Ya know what, I actually kind of love this idea?
All perception reasons aside, I actually think tons of MMA fans would want to see this. It would do the same thing for the sport as it did when Brock first jumped in, it would bring new fans.
They could market it as a light-hearted freakshow fight, almost a distraction from all the crap going on right now with PED's. Not a main event, maybe a co-main.
That leads to my next point: How much credibility could the UFC gain by putting a clean-of-PED's Lesnar back in the octagon? Let's assume for the sake of argument that most peolpe think Lesnar is on the juice. If that is the case, and they could show that Lesnar is not on PED's (through tests and obvious body changes), it doesn't even matter what the outcome would be. Mir wins great news for the UFC's new anti-PED program, cheaters don't win when they're off the stuff. Lesnar wins, there ay go, the dude is a monster, back in full swing, sign that guy up for a new contract, the HW division needs something special anyway.