It was the opposite: He was useful in all but one series. He was good against the Lakers the first three games. His mobility really helped against the Warriors. And his post defense came up big again in Memphis.
Splitter is a fine offensive big in the PnR. He may even be the best in the league at it. The only thing that can limit it is its predictability. If Splitter got a jumper, then the defense wouldn't know where he was going to go after he set the pick, and the help would be a lot slower on his rolls. There are tons of reasons why Duncan was better in the pick-and-roll than Splitter, but the biggest one was him having a reliable jumper.
None of that excludes him getting post moves, though. He certainly is still young enough to develop those, especially now that he has two new big-man coaches.