Making Trek just dark would obviously be heresy, a word I do not hesitate to use. But, to use TUC and FC as examples, it is perfectly fine to have characters that go to the dark side but travel back before it is too late. This creates a lovely form of compressed dynamism, i.e. paradise is not something static that just is and always will be the way it is but it is constantly in danger and everybody has to work hard to maintain it. It might seem fairly static on the first glance but on the second glance you realize that without constant efforts it would just collapse.
"Vigilance, Mr. Worf, that is the price we have to continually pay." Taking Picard's line out of context one could easily substitute vigilance with discipline.