While they are of course the lead roles, I don't think TOS is really about Kirk and Spock as characters. They, like the supporting cast, were really only there as vehicles for moving the stories along and debating the moral dilemmas those stories brought about. It was not a primarily character-driven show, but a plot-driven one.
Without making any kind of judgment about which is "better," I will say that this stands in contrast to most of the movies including the two Abrams efforts, where the plot basically exists to develop the characters and give them something to do, and that's about it. They involve stories designed to tell us about certain characters, rather than characters designed to tell us stories. Not that this goes without exception for either the movies or the show, or in many ways isn't simply the natural result of changing conventions and of the characters becoming more familiar after decades of seeing them in action, but I think it's a generally important trend to recognize.
If you watch the television show Star Trek
, you probably will
get a good sense of the characters of Kirk and Spock, but to characterize the show as being about little or nothing more than these two characters and everything else as window dressing to showcase them is to misconstrue things a bit IMO. The show was an exploration of human nature, culture, and conflict, and the characters including Kirk and Spock were just there to bring us into the stories and keep us from getting out of our depth as they went along.