"I'm not Luke Skywalker!"
I think Mark is in the same boat with Shatner and Nemoy. If you become too associated with a character, it typecasts you as being "only able to play that character. (Look at Joey Lawrence from "Brotherly Love" and "Blossom" - exact same character - different last name)
I know it may have been a joke on Mark's part when he appeared on "Just Shoot Me" as himself and had to fight off drooling fanboy Dennis Finch, having to endure questions like 'What was it like flying through space with with Chebacca' [paraphrased]. It also may have been a joke in "Jay & Silent Bob Stike Back" when he gets his hand chopped off and whines "Not again..."
I think Mark appreciates the fact that his fortune was built with Luke's lightsaber, but at the same time he wants to be able to do voiceovers like Ripburger's and smile as people say "WOW! That's the same guy who played Luke Skywalker? It doesn't sound like him. He's a really versitile actor!"
Unless George makes Episode 7, Mark can safely hang up his Jedi robes and make a living without having to suckle at the Lucasfilm teat to survive. The fact that LEC hires him to voice MANY other roles doesn't act as an apron string for him, but rather shows that LEC knows a good voice actor when they hear one.
Print and submit this signature for a free breakfast with the works* at The Kickstand
* The "works" is a beating so intense, you'll pray for death.