I guess I'll chime in with my experience. I made NorthernBrewer's Scottish Ale 80+/-, but played around with the hop schedule and found the finished product to be bland, so I added a half a cup of scotch to the bottling bucket to try and make it a little more interesting.
At first taste, I was horrified at the pure **** I had created. Truly tasted awful. A month later though, it was really really good. I guess the flavors just had to mesh together a little bit more.
Anyway, I wouldn't make the beer again, it was excruciatingly, painfully, uninteresting and boring- even with my additions. Maybe the All Grain version is better? Anyway, although my flavor addition did seem to help the beer, I feel it is wasteful. I would much rather drink a scotch and drink a beer than drink a scotch in a beer. Besides, if you think about it, it's sort of assbackwards. Both historically and flavorwise, it makes little sense. Why am I using such a finely crafted product (even cheap hooch has a carefully guarded recipe) that has spent so much time aging and undergoing carefully controlled distillation, and then adding it back to an undistilled fermented beverage? It just doesn't make any sense... I can't imagine why you would do it except to up the alcohol percentage.
But with that said, I think the addition of the base ingredients is awesome... flavored wood (or soaked cubes to emulate it), fruits, zest, sugar, etc.... that's really cool. Of course, historically, everyone is interested in flavor and taste and alcohol, so it only makes sense to add those things and I think some of the results are awesome.
But adding distilled alcohol just strikes me as bassackwards. I've done it, but I felt kind of silly about it afterwards... But of course, that's just my lesser fraction of a nickel...