Hey, Brando did it. And Sean Connery, in Darby O'Gill and the Little People.
However, Patrick Stewart got his start from the Royal Shakespeare Company. He does a one-man Christmas Carol. (My high school California history teacher, who was also the drama critic for the local paper, got to interview him for it--he performed it at CalTech!) He is a great dramatic actor. While I think he's lucky--he isn't just Captain Picard anymore, especially now he's been Professor X as well--as Star Trek actors go, I can also see his point. If you're a good actor, and he is, there's no real joy in being the same character over and over for decades. Heck, you got into acting in the first place so you could be all kinds of different people.
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"