Creative incest this would have been.
Having the same director for both Trek and Wars is already enough. Do they also have to share the actors? That's silly.
There are 100,000 unemployed actors in Hollywood. Give other people a chance.
If they're right for the part and do a good job, who cares?
We must keep the franchises pure! We must ensure the purity fo the Trek above all else! We must stand against the dilution of true Trek...
...sorry, had a "moment".
Reality: You're right. I don't care what someone has or hasn't worked on. So long as they're the right man or woman for the job, let them have it.