actually, I've seen one game with quite a few giltches that I'd consider 'positive'.
Namely, in the game called 'platform racing 2'.
The game itself was only fun for so long, as most levels were pretty much a variaton on the rest.
Glitches, on the other hand, were tricky to pull off, allowing for much more challenging levels.
While unintended initially, they provided so much, that they never got fixed.
Of course, during the design of a level (there is an editor), you'd have to make sure that glitching through it isn't possible.
As for the reason glitches are rarely 'positive' for those it happens on, is that they are kept 'secret' untill fixed.
If a person unaware of a glitch faces somebody who randomly gets a positive effect, he probably thinks he faces a cheater.
If that person would bump into a glitch that set him back, he'd more likely recognise it as a glitch.
