This is Dom's competition. Nobody else.
The Phillies haven't been completely clear on their reasons for demoting Domonic Brown to AAA this year, but the reason why they should have demoted him is to have him work on his defense there. Defensive metrics are imprecise, but all the data that was available in 2011 said that Brown's defense was really bad. That's why his 2011 fWAR was 0.0.
Iron Pigs beat writer Michael LoRé recently tweeted that Brown has looked good in the field so far in 2012 (h/t TJE1343). This should be taken with a grain of salt since it's only one guy's opinion, but it's an encouraging data point nonetheless. Maybe Brown has solved his problems.
But that still doesn't mean he should be brought up immediately, and no it doesn't matter that the Phillies' offense is struggling in his absence. There are three inescapable facts that people need to grapple with here.
1. Brown is a LF.
2. John Mayberry, Jr. is a LF, though he can also play a little 1B.
3. Ryan Howard is probably going to return in a few weeks.
Which means that if Domonic Brown is here when Ryan Howard comes back, the Phillies will have three guys occupying two positions. I will refer to the Pauli exclusion principle to suggest that this will be a problem, and not necessarily one of them good problems. It will have one of the following three results.
1. All three guys will stay on the roster, but one or more of them will start less than 70% of the games.
2. Brown will be re-demoted.
3. Mayberry will be demoted.
Here's the upshot. It isn't completely unreasonable to want Brown to be promoted ASAP, but if you do, then you necessarily must acknowledge that his competition is Mayberry, in something approaching a zero sum game. Laynce Nix, Ty Wigginton, and Juan Pierre are convenient targets, but in reality they don't have much to do with the situation. If you want Brown to play in 2012, then you want Mayberry not to play, and vice versa. Of course, Brown and Mayberry could play alongside one another for the next few weeks until Howard returns. But the numbers crunch is coming. Everybody and their mothers can foresee it. To ignore it would be foolish.
That's especially the case when you consider that every single time I see people complain about Brown's demotion, they're just as upset about the Phillies having "jerked him around" by ping-ponging him back and forth between the big leagues and AAA over the last two seasons. But calling him up to the majors now would just set him up to be "jerked around" yet again in a matter of weeks. The only way to avoid it would be to take away Mayberry's starting job. That wouldn't necessarily be a crazy solution, but again: if you're going to argue that Brown should be called up and kept here to stay as a regular, then you also need to argue explicitly that you do not want Mayberry to play regularly.
2013 is a different story. Any number of things can happen between now and then. Shane Victorino might leave via free agency. Mayberry or Hunter Pence could conceivably be traded. Who knows? But that's just all the more reason not to precipitate a numbers crunch prematurely, when a little patience might make the crunch avoidable altogether.
My opinion? I'm moderately skeptical that Mayberry will be able to hold down a starting job and prove that his great 2011 wasn't a fluke. And I'm fairly confident that Brown will be able to correct his fielding issues eventually (I'm very confident that he'll be good at hitting). But Mayberry hasn't yet had a fair chance in 2012 to win or lose his job. The Phillies should wait at least another month, if not longer. If Mayberry plays well over that period, then Brown should stay at AAA and the Phillies should work on finding a way to get him into the starting lineup next spring. If Mayberry doesn't play well (and Brown's defense is fixed), then Brown can be recalled. In the meantime, we should all just grin and bear seeing Nix, Wigginton, and Pierre start games for the next few weeks. There are worse things.