From the network's point of view, it's all based on demographics: If you can get 17-35 year old males with the scifi thriller elements, and a similarly aged female audience with the chick flick elements, well, you've just doubled your potential audience.

From an artistic point of view, it is a worthy goal to weave these disparate elements together in a cohesive, dramatic whole. After all, life does have mystery and conflict and thrilling parts to it, and people do have human aspects and social relationships. It gives the writers, directors, and actors a real challenge: convey a dramatic narrative and also make the characters real, live, breathing people.

At its best, the richness of the characterization can complement the narrative and vice versa (think BSG). At its worst, it turns into a muddled mess (think Grey's Anatomy).

From the viewer's point of view, it is good to have one's presumptions challenged. A scifi thriller shouldn't just be hardware and space battles with a sexy babe thrown in — you have to care about the characters too. A chick flick shouldn't just be all about emotions and relationships — there has to be a narrative and that elevates it from mere soap opera.

To date, the jury is still out on Defying Gravity, since only three episodes (well, a 90 minute pilot, plus one episode) have aired. The creators seem to have enough talent to be able to pull it off, but we'll just have to see.