Adding corn sugar or, preferably (since it's cheaper), regular old table sugar, is completely appropriate for a lot of beer styles. While it DOES boost alcohol content, where you're really looking to use it is beers where you WANT a really dry finish.
For example, a lot of Belgian styles use sugar, because they're MEANT to finish bone-dry. A recipe I'm going to be brewing next week is for a Belgian Golden Strong Ale... supposed to be a lot like Duvel. All it is is Pilsner malt to about 1.047 OG, and then 3 lbs of table sugar. Yep, 3 lbs. Sounds like a lot, but Duvel finishes damn dry, and the only way to get that dryness is by using table sugar. The Pliny the Elder clone recipe I made a few months ago also used almost a lb of table sugar, because a lot of IPAs are meant to finish dry as well.
And don't be too taken by a lot of people arguing that sugar gives a cidery taste. I think a lot of "experts" have agreed that that isn't the case.
Would I use sugar in my beer just to boost alcohol content? No, but I would in styles where you want some extra alcohol and a nice, dry finish. I should say I also brewed a Belgian Tripel last year that had 2.5 lbs of table sugar, and it didn't taste cidery or off at all.