Purvis isn't a great defender, but he has the ability to be, and he showed more flashes last year than any of our other wing defenders. Steals, and especially blocks, are not a very good indicator of effective perimeter defense. Instinctually, he's not Boatright or even Dyson, but he has a strong frame that helps him navigate screens and wall off on drivers.
And yes, we were 4-4 in March (4-3 if you don't include the NIT game sans Boat), but that stretch also coincided with a much tougher portion of the schedule (SMU twice, Cincinnati, @ Temple, Tulsa, ASU and Memphis - South Florida was the only gimme) and included two of our best wins of the season (SMU and Cincinnati).
Now, that being said, I have Purvis 6th on my list. He has flaws. I probably err closer to your assessment of his game than I do somebody who thinks he's going to be a first team all-American. I think it's a little disingenuous, though, to act like his transformation towards the end of the season was a "false prophet" that didn't help us win games (and it wasn't just his scoring that improved, he got better as a playmaker, too). Hamilton and Brimah both fell of a cliff offensively, and Boat was evidentially playing hurt in the AAC championship.