Hit from a tanks perspective servers two purposes.
1. Hit is a DPS stat, by increasing the DPS of the tank, you are increasing their threat generation.
2. Hit improves the chance of your abilities hitting the target, in particular taunt.
Certain fights require you to taunt in a timely fashion and do so reliably without missing. In those cases a tank may choose to wear additional hit gear, or they may simply just opt to use their second taunt if the first fails (provided it is acceptable, wouldn't be acceptable for a warrior to hit Challenging Shout to taunt Gluth in Naxxramas, cause any Zombie Chow as well within range would also wonder over and heal the boss).
In general, so long as you generate more threat than the DPS (and healers, though typically that won't be the problem), hit is fine. The tank serves two key roles in a raid.
1. Piss the thing off more than anyone else so it doesn't go and play whack a gnome with your DPS. (Threat Generation)
2. Survive being able to piss it off. (Survivability)
You gear for threat and you gear for survivability. If your threat is fine, work on survivability. If your threat is falling behind, work on threat. That's all you gear for, the rest is technique.
One additional note, avoid going above 8% melee hit chance (10% spell hit chance) because at that point many of your abilities will no longer miss. Some abilities like taunts may still miss because they may follow spell hit rules. Though it still wouldn't be advisable to exceed this amount of hit.