I tend to hold my gloves in a high guard, mostly because I don't like.. you know, getting hit.
However, when I'm moving to punch, my hands come down, unless I'm jabbing, which I can manage to throw pretty crisply from that high guard.
The weakness, to me, is that its awkward. I cant generate power from having my hands in front of my face. Mike Tyson could, but he's mike fuckin tyson, he could generate power punching in zero gravity.
Personally it's not how I fight. It's a shitload of head movement, it's tiring, and it saps a lot of energy. Tyson pulled it off well because he was so gifted and it FIT him- a smaller guy, crouching, his gloves in front of his face, going side to side = very difficult to hit.
I would be very wary of trying to emulate anything of his style. Unless you have his physical attributes, I think you're going to get tagged up pretty badly the first time you use it against a good fighter. It's like trying to fight like Roy jones.