That is also possible. Either way, I am comfortable saying that Roethlisberger's 34.3 passer rating IS indicative of his play this year.
All in all, I think QB rating is not a bad tool. It certainly isn't the worst of the generally accepted statistics. I do not know the formula off-hand, but I do know that it at least tries to account for a wide range of parameters to give a balanced view of performance. However, not knowing the formula, I cannot say if it takes down/distance/situation/etc into account. That information is CONTEXT and it is CRITICAL to any statistic being worthwhile.