How about Eli? He has 57 career starts, yet he has never had a QB rating over 78. This year his QB rating was 73.9. Exactly the same as Rex's in 2006. They both also had identical TDs at 23, and INTs at 20.
Vince Young, career QB Rating of 69.0, yet he has started two years straight.
I am not defending Rex, but there are a numerous QBs who get chance after chance to prove themselves. Harrington and Kerry Collins also fit that mold even though they did it with different teams.