Only about 10% of the general population is left handed - but due to some opportunistic selection and training choice somewhere just north of 30% of major league pitchers and around 35% of major league hitters throw or bat from the left side. By example 10 of the 14 pitchers on the current Phillies active squad throw from the right hand side while 4 are lefties (Alvarado, Strahm, Suarez, & Vasquez). Among Phillies batters there is balance: 8 bat right handed, 8 bat left handed (including Harper). Generally the same handed pitcher has an advantage. This is due in part to visibility - it is harder for the batter to see the motion of the ball in the pitcher's hand when thrown from a same handed pitcher. There are also ball motion advantages for the same handed pitcher. However, some hitters hit same handed pitchers for more homers, but much lower average (like ALec Bohm) and a few hitters (often LHBs) have odd "reverse splits" where they hit same handed pitchers actually better than they hit the opposite (including Phillies Stubbs, Stott, & Maton). But in general, since there are almost twice as many right handed pitchers - left handed batters have an advantage over right handed batters - and since learning to hit left handed is considered a training decision, many young players (regardless of natural handedness) learn to hit left handed if they can and some teams (like the Phillies) stock up on left handed hitters.
Methodology - I used the Baseball reference splits and looked only at OPS as a measure of hitting.
To avoid single season anomalies and get a broader sample I looked at this
OPS vs LHP = (2022 vs LHP OPS * .5) + (2021 vs LHP OPS * .3) + (Career vs LHP OPS * .2)
OPS vs RHP = (2022 vs RHP OPS * .5) + (2021 vs RHP OPS * .3) + (Career vs RHP OPS * .2)
Diff = ABS(OPS vs RHP - OPS vs LHP)
Looking at two seasons was interesting because we have at least two players - Turner & Castellanos (you could also say Harper) - for whom their 2022 stats were a drop from their 2021 stats. It doesn't really matter to this analysis, but it was interesting. Of course some folks (Stott, Hall, Guthrie) only had 2022 and I just used those, and some players only had two seasons - I used the formula and got some weighting off the career numbers between the seasons. I performed these calculations for the 16 players which are likely to see significant playing time for the Phillies (as of now) including: Bohm, Castellanos, Cave, Guthrie, Hall, Harper, Hoskins, Marsh, Maton, Realmuto, Schwarber, Sosa, Stott, Stubbs, Turner, & Vierling
The goal of platooning is to protect the team's offense from players who just don't hit LHP or RHP well. So the priority should be not who has the biggest differential, etc. - but just who doesn't hit LHP or RHP well.
Lowest OPS vs. RHP:
Stubbs (.606), Bohm (.618), Stott (.623), Sosa (.657), Cave (.659)
All others hit at least .700 OPS vs Righties
the small sample of Guthrie (13 PAs) - OPS 1.205 was anomalous
Hall (.865), Schwarber (.958), and Harper (1.000) were exceptional
Lowest OPS vs. LHP
Hall (.167), Marsh (.515), Cave (.525), Sosa (.662), Schwarber (.676)
all others were at least .700 vs. lefties (inc. Stott)
Hoskins (.936), Stubbs (ss/.928), Turner (.925), Bohm (.881) were the best
I don't think the differentials are that important, but obviously Hall had the biggest (.698), followed by the small samples of Guthrie (.405) and Stubbs (.323)
Castellanos, Realmuto, Sosa, Vierling, & Stott have small differentials (< .100) implying they hit equally as well against LHPs or RHPs.
What can we interpret?
Nobody really needs platooning vs. RHP. Although Bohm shows up as the second weakest Phillie by OPS vs. RHP, he hit 10 of his 13 HRs vs. RHPs - you don't want to take him out vs. RHPs because homers are part of his game. The others are backups or Stott (who has an odd reverse split) and might be considered a small sample as his hitting improved over the season and his split differential is small.
Vs. LHP however it's obvious that both Hall and Marsh have limitations.
The sample may be small for Hall (12 PAs) vs. LHP but it's small perhaps for a reason. In any case it's dreadful. As anyone can be shuffled around to DH, the first backup on the list might be the obvious candidate - but that would be Maton (.834) - who's career sample vs LHP is 65 PAs - but he is a LHB and I'm not sure that makes that much sense - counting on an anomolous reverse split. Next would be the even smaller sample of Guthrie (.800) who is a RHB at least. But the logical choice is Vierling who has a respectable OPS of .769 vs LHP (based on a good sample), is a RHB and natural outfielder to replace Castellanos or Schwarber (who would move to DH).
But the other problem complicates this. Marsh also has problems with LHPs. The backups at centerfield are Vierling and Jake Cave (who may not make the 26 man roster). Cave is as bad vs. LHP (very similar stats to Marsh) so that leaves Vierling. I guess Cave was acquired to play for Marsh if he has injury list time like he did last year when we had to pickup Bradley Zimmer for a ten day cup of coffee. Anyway it makes sense to platoon Vierling for Marsh against LHP. But then Vierling can't platoon with Hall so we are looking at someone else from the bench and that could be the aforementioned Maton, Guthrie, or Sosa (.662 vs LHP). If you go for the smaller sample warriors of Maton or Guthrie - they could still play outfield and send Castellanos or Schwarber to DH. If you go for Sosa, you'd want his glove at third and then either DH Bohm, or play Bohm at first and DH Hoskins. As Sosa is perhaps the best defensive Phillie with the glove (esp at short or third) and Bohm is perhaps the weakest Phillies glove among starters, this latter option has the added advantage of improving our defense. This makes the most sense to me.
The argument against platooning Marsh? If we wind up swapping him for Sosa we only gain .15 or so in OPS in that swap and both hitters hit much better in their Phillies second half than their first (or prior career). So it's harder to tell what's going on with either of their hitting (real change or temporary bump?). Also, Marsh is the youngest Phillie (other than Maton) and the long term hope in CF. "Giving up" on his hitting lefties doesn't help him develop against them and could doom him to a career in platoons. So at least in the early part of the season it may make some season to have Marsh "hit through it". (the same logic does not apply to Hall because Hall is a bit older and his role in temporary in nature anyway as he is only there to hit for the injured Harper - "fixing" Hall's much worse split is not a major league problem). One could argue that even though Marsh is a LHB & Sosa a RHB - there isn't enough of a clear benefit and asking Marsh to hit vs. LHP could improve his hitting vs. lefties and by the second part of the season Harper will be back and we'll won't need a platoon for Hall and if Marsh is not improved - we can platoon him with Vierling. Vierling does hit .769 vs. LHPs and Sosa .662.
Another option is to get another bat. If we could find a batter (position doesn't matter) that hits well above .700 vs LHPs on a cheap, one year deal, then we could platoon them with Hall (& Vierling with Marsh). As Vierling is the platoon for Marsh and can play most of the positions Maton can - Vierling would be the bench and Maton might have to go back to LHV. By the time Harper gets back we can dispose of this batter or find a slot through Dombrowski's magic with trade deadline deals and/or the expanded roster (which is only two players these days).
My conclusions - with what we know now we will start the season with Hall at DH and Marsh in CF vs. RHP but we should platoon them both vs LHP. My recommendation is to platoon Marsh with Vierling and Hall with Sosa (at 3rd) moving Bohm to DH.