I've done this many times. I don't think tripping whilst drunk is a "waste" just a different context really. Its more of a hedonistic experience then profound, but I have found that LSD whilst drunk is still intense and thought provoking. The main concern though is: if you have lowered inhibitins due to alchohol and feel kinda blase about dropping acid, you may really regret it in a few hours when the booze is wearing off. Also- a hangover beginning during a peak is pretty damn shit. Nothing wrong with having a few drinks to take the edge of the experince, but you need to aware that acid lasts a whole lot longer then alcohol.
Personally, as a virtual non drinker, this issue doesn't arise very much. However, if I was to take a psychedelic whilsts drinking, it would be 2C-D or 2C-I, both of which have entertained an intoxiated me many times before. In fact, if I take 2C-D by itself, I often end up having a few beers or some whiskey....