The C and PG positions have been the Spurs' defensive weaknesses, from a statistical standpoint, so far..those are the two positions that are affected the most by Splitter's defensive presence, of course..
His value is evident by any advanced defensive metric you choose, and his work against Dirk and Aldridge was phenomenal in last year's playoffs..
Diaw is more useful in some series' and Baynes is shaping up into a nice 4th big, but the Spurs have absolutely no chance at winning a le without their best defensive big and best pick&roll defender..