He's able to hold position near the rim, and is almost impossible to post up against. None of the other bigs can really do that. Poertl is great at rebounding because of his positioning, and Babe at blocking, but neither really have the physical presence. JV is NOT good against a really mobile big though.
Regarding Ibaka, I have to think we wouldn't have signed him if we had known how good the young players were going to be.
He also gives us that inside, more traditional scoring threat. I think he's under-utilized in that role.