There's an element of truth to both sides here. Character without substance doesn't advance the educational goals (damn, I sound like a principal) of mock trial. You can pull any kid out of your theatre department who can play a good doctor, but that's not going to help you if they don't know what to emphasize, when to pick a battle with the CXer and when to gloss over, what specific wording to use to avoid objections, and so forth.
Like SkySoldierEbG says, there's no reason to stifle creativity on the stand, or do anything else to prevent witnesses from being memorable. At the same time, character for character's sake and nothing more can be cliché and annoying. I've been doing this for 10 years, and I just might lose it if I hear a British-accented physician for the fiftieth gabillionth time.
Candace (as usual) hits it squarely....mock isn't the same as real. There are plenty of things AMTA
could do to make it more real, the question is do we really want to go there? We could make all of these jury trials (at least in name and procedure, not necessarily with an actual jury...that I support), we could have motion
in limine hearings before each round, we could stop scoring witnesses altogether and place the focus squarely on the legal elements (not something I support.) I think on the whole most people are pretty content with where things are right now (except perhaps for The Elusive Third Bid

)