ExploderWizard
Hero
Well, obviously classes and skill systems are not mutually exclusive. I happen to think that a skill system is the best way to model a lot of sci-fi tasks (research, hacking, medical treatment, scanning for life signs, etc.) regardless of whether those skills are specific to individual classes.
But there are probably sci-fi genres that could do without.
I generally disagree with the mutually exclusive sentiment. Games featuring classes are better off without a fiddly skill system tacked on and games with a good comprehensive skill system have no need of classes in the first place.
I can enjoy a good game of either type but find most hybid systems to be irritating to some degree.