I think Russel T Davis has taken a risk with Tate, he risked upsetting fans of both Dr Who and Catherne Tate. Billie Piper was also not well known for her acting (just an brief POP career) he took a chance with her, and it payed off.
Cahterene tate from what I have seen is a brilliant actress (she has done some straight roles) and after watching the comic relief Dr Who sketch I actually think she was hired on her merits.
I think overall she's shaping up well. The hoo hah and publictiy hasnt hurt Dr Who and that to me is what were looking for. I dont think Russel T Davis would risk his reputation looking for stars who are bad actors.
even for the publicity. Just my opnion!