Well in that regard, I think you can distill it down to the point where you can say that just about any major "thing" that happens is going to benefit a relative few a great deal, but the general population can still get a lot of benefit as well. Example: Facebook (and most other major companies) made a few people very rich, but many other people enjoy the product. (In the case of Facebook, myself excluded.)
Back to the Revolutionary and Civil wars, while it's true that maybe relatively few held the genuine interests which started them, I think we can all agree that after they were over, the general population benefitted from the result.
That's the point I'm trying to make. Maybe another way to say it is, even if a war is started for "the private interests of a very few", it doesn't mean that the good results (assuming there are good results) won't benefit the population as a whole. And if it does, then it could be argued that the population's interests are served as well.
"Just because you've always done it that way doesn't mean it's not incredibly stupid." - Despair, Inc.