We've always played two bigs - until the last couple years when Roscoe and DD/Niels saw time at 4 out of necessity based on personnel. Rudy never played the 4 here and Sticks was almost exclusively a 3, except very rare spot minutes at the 4.
Right now for 2014-15 (assuming no Boat or DD), we have five guys to rotate at the 1-3 (Cassell, Samuel, Calhoun, Purvis, Hamilton), and five at the 4-5 (including Leon). Perhaps someone doesn't get in the rotation, but even then, we're still a solid 8 or 9 deep, where one injury doesn't kill us. I don't think Hamilton or Calhoun could realistically be a stretch 4 even if we wanted to (they don't have size and/or strength of Roscoe or DD), so if one of the 5 guys at 1-3 get hurt - we're fine, we split those 120 minutes four ways. If two guys get hurt, the remaining three have to play heavy minutes and Leon becomes our emergency back up 3. Not ideal, but not many teams can handle two major injuries without some issues.
We don't have the same quality depth up front on paper, so an injury to a big would probably force a player or two into more playing time than they are ready for. That's been an issue for us since Adrien/Thabeet left, though.
Now, if DD leaves, we won't have a hybrid 3-4 (other than perhaps an improved Leon, but that's asking a lot), so Robinson would clearly make us more flexible and improve our team, where we wouldn't have to force a raw big man into the lineup if he wasn't ready. So there is a need for him and I hope he comes, but if not, there's no one injury that would put us in severe trouble.
