I think the point is the imagination aspect of it. Basically, whether we admit it or not, we've all conceived the Star Wars Universe in our heads based on the movies, books we've read, etc. That's why when something comes along that doesn't agree with, or seems out of place with, the universe we've individually created, we don't like it. So George Lucas could decide Bossk has the voice of Duane Chapman (Dog the Bounty Hunter), but that won't necessarily jive for some. Essentially the further the Star Wars universe is fleshed out in movies, books, comics, games, etc, the less there is to imagine on our own...
So, sure, they could have him speaking basic, which will be fine to a lot of people (as David mentioned there is already a Trandoshan that does).