Who says directors and actors can’t be the same people? Peter Bonerz, the dentist from The Bob Newhart Show, has directed a ton of television. What’s-her-name, who played Naomi on Mama’s family, has a lot of directing credit under her belt. In the movie world: Clint Eastwood – actor and director. Woody Allen – actor and director. Barbra Streisand – annoying celebrity. (Ooops – guess she doesn’t fit in. ;))
Anyway, actors can make GREAT directors, because they understand the craft of acting from actually doing it. (This is not to say that all good or great directors must have been actors. That’s certainly not the case.)
As for the Star Trek universe, well, there’s lots of backstory, lots involved in understanding the setting, and the characterizations. Having played a character in that universe seems to me to be a positive thing for a director of one of those shows. They understand the visuals, the attitudes and sensibility of the setting.
Cheaper than “real directors”? Not sure what that means. But certainly able to more easily slip into the setting and make it meaningful without having to have a major briefing or having to read some sort of guide to the Star Trek universe.
Having said that, I don’t know how they are as directors (well, except for Frakes and Burton, because they directed on TNG, which I do watch), because I’ve never seen an episode of Enterprise.