I mean, having one or two characters switch sides, maybe even twice, is okay, but doing it all the time, and to just about everyone (I don't even know how often Noah and Nathan have switched now, and Sylar, and... yeah. way too many characters.) just makes it stupid, and has me trust nothing any character does any more, cause it will all be negated three episodes later, anyway.
Yeah, that thing in X/1999 was really really stupid. It had no foundation in the character whatsoever, it was just... possibly necessary for the plot, and, well character "development" shouldn't work like that.