The value of a player is much debated fact but there are some things that are often lost in the discussion. Here I am going to try and show the distinctions between different types of pitchers and how they are valuable to a team.
To make this comparison I am going to use three measurement tools here WAR, the 20-80 scale, and the player "role". The idea of WAR is to provide a direct comparison between players, and assess their value to a team. The 20-80 scale is to provide distinct categories between different types of players and direct correlation to their minor league ceilings. The same is true of the "role", this is often the most confusing because the majority of media will describe a player incorrectly. The important thing is that these can change at a moments and while I will describe these as distinct categories it is really a bell curve and there is a rapidly decreasing number of players at the high end.
The biggest thing to remember is that these starter distinctions have nothing to do with where they pitch in a major league rotation. There are definitions of the pitches and tools a pitcher must have to fit these designations, but I will pass on these for now.
#1 Starter (80-75) - Synonymous with ace, much like superstar this is an earned designation. Even within this category there are sharp designations at times
WAR projection - 6+ wins
#2 Starter (70-65) - Often #2 starters are confused with aces, they often have a great profile, but one thing that makes you pause and say if only something was a little bit better. If this is an outcome for a pitcher it is a gigantic developmental success for the organization. To best illustrate this distinction the 3 pitchers that toe the line between #1 and #2 starter for me are Zach Grienke, Cole Hamels, and Matt Cain.
WAR projection - 4-6 wins
#3 Starter (60-55) - The majority of opening starters were #3 starters. A #3 starter is not an average pitcher, rather it is an above average outcome. The going rate for a #3 starter is $11-$15 million a year. This is a very broad category of starter with many different definitions but ultimately a #3 starter is something that you want in your rotation.
WAR projection - 2-4 wins
#4 Starter (50) - Most rotations are comprised of #4 starters, and it is a major league average player. There is an acknowledgement with a #4 starter that they are not dominant but
WAR projection - 1-2 wins
#5 Starter (45) - Ultimately their best quality is that they can pitch multiple innings on the major league level. That in itself is a very valuable thing as the average middle reliever is normally a worst pitcher than the starter.
WAR projection - Hopefully greater than 0
Relief pitchers range from fungible to very valuable. It is important to realize that value is relative. Most good relievers will be worth less than 2 wins a year, and many will be worth close to 0. However, there has to be a realization that a middle reliever is a replacement level player so anyone brought up is below replacement level for the most part. A really good closer who can deliver dominant innings and especially a high strikeout rate can be worth as much as 3 wins a year, and while this is less than many #3 starters it is much more than a poor reliever can give you with those same innings.
What This Means:
The gap between a #1 starter and a #2 starter is as almost as large as the gap between a slightly above average starter and a replacement level player. This is the same between a #2 and #3. Aces are incredibly valuable players.
If a player has a #2 ceiling that is incredibly valuable and if your farm system can produce that it is incredible
Same thing for #3 starters, no prospect writer is insulting a player when they say he is a #3 starter. A rotation of #3 starters won't carry you to playoffs but if your 4th and 5th starter are #3 starters your team is going to be really good
A #5 starter is better than a middle reliever and those innings are very useful
Relievers are often said to be fungible because they have a high turnover rate but 0 WAR is better than negative wins
It is all about marginal value, the difference between player value is the value not the overall value