I think there's a bit of a danger bringing in well-known actors for major roles on Trek, especially well-known genre actors. You could divide your projected audience before you've filmed a single frame.
It would depend on how the actor/actress was used, IMO. Someone like Glau or Lumbly would be great as a guest star or recurring character (like Whoppi Goldberg) but not as a main character for the exact reason you mention. One of the things I like about Trek
is the show's ability to elevate previously unknown actors by associating them with its brand name. I had no idea who Michael Dorn was before I saw him in TNG; now, I'd recognize him in just about anything he did.