Over at THE BOOK, Tango responds to a common complaint about FIP (that it's not a complete picture):
OBP focuses on the skill to get on base, and thus treats a HR equal to a walk, even though we know it’s ridiculous and incomprehensible to do so in the grand scheme of things. OBP needs to be paired with SLG to get a more complete picture. It’s easier to stand on two legs than one.
FIP focuses on the pitcher’s skill on events that don’t require his fielders, and does not consider the sequential timing of those events (i.e., with men on base or not). To get a more complete picture, you’d like to know the pitcher’s skill on batted ball events and his skill on sequencing the events.
But there's a caveat:
FIP is a HUGE component of pitching, enough that even if you don’t consider the other two major components of this tripod, it still has enough strength to stand on its own to tell you a lot, even though a lot is not everything.
What Tango is getting at is that Fielding Independent Pitching is clearly not a complete picture of pitcher ability, but that it is strong enough to stand on its own.
The question now is about batted-ball and sequencing skill and which pitchers benefit the most from the latter two legs--and whether or not these necessarily affect our general view of pitcher ability through the FIP lens.
Continuing with the stool theme, isn't there at least one more multi-faceted leg? What about durability? How much does pacing figure in (and does it relate to durability)? How long can a pitcher last in a game? How many pitches is he good for? Can he make it through an entire season without blowing out his arm? How can we take each, weight each appropriately, and get not only a complete picture of a pitcher's ability, but also put it in context of his peers?
And do the answers to each of these change what we see when looking at FIP?