I was just responding to where you said - "you can put all four of them on the floor at the same time - Purvis, Hamilton, Robinson, a point guard like Cassell, and a center". Calhoun is not on this hypothetical floor.
I could be wrong but I think Purvis is probably getting star minutes - 35 mpg or so - to make sure we have enough playmaking/ball handling on the floor at all times. If we get someone like Cassell, and Samuel is good enough to be a backup, then Purvis will probably be slotted for almost all of his minutes at the two. Now things get more crowded - Calhoun is an incumbent three with Hamilton pushing for minutes there already. If Purvis becomes the PG, then things open up a lot, but getting involved with Cassell, Lyle, Snider, Perkins, et al, is a pretty good indication that we don't think of him as a PG.
My primary point is that if Robinson sees himself as a pure three, wants to play the three in college to help his chances of being an NBA three, right now we're not in a good position to get him, since we have two guys who's best college position is the three here already. He'd likely be asked to see most of his minutes at the four in a DD role (and DD could be back next year too), forced to guard post guys at 6-8, 180 pounds. He may not see that as a great fit for himself, even if we think it is. If he is content getting out on the floor however he can, being a stretch four, and is a "whatever it takes to win" type of guy, then we'll have a better chance. I honestly don't know if he's that type - kids ranked that high think about getting to the next level first and foremost, and it's hard to blame them.