Someone will doubtless correct me if I'm wrong, but as far as I know going above the hit cap has no benefit. Going above the 'expertise cap', as be imba calls it, does though - that cap is the soft cap that prevents a boss from dodging. However, the boss can still parry, and more expertise lowers his chance to do this - and that's even better than lowering dodge, because if a boss parries, his next attack will come sooner, and can easily be a killer.
If you're talking strickly tanking, over 9% hit has no additional benefit for a tanking warrior.
Going over 5.6% expertise has HUGE benefits, for one thing, dodge cap seems to be found to be 6.5% from Elitistjerks so it isn't even the softcap anymore. secondly boss parry mechanics range anywhere from 8%-15%. So there definitely is more to go in terms of parry reduction from expertise, which likewise continues to act like +hit for threat purposes.
BeImba is an unreliable source. It's a good place to go and get a quick look at your gear, but it cannot be used as an end all source for upgrades. For one thing the administrator has been asked multiple times by multiple credible sources in the warrior community to fix the 490 defense "cap" and 5.6% expertise "cap" that the evaluation spews out as useless, when in fact, it is not the case.