Your correct, but the melee swings can be mitigated through armour and avoidance, the bleed damage can not. If the DPS are under-performing, popping your shield wall when the bleed damage begins to get uncontrolable will buy you precious seconds to live. I am assuming that as a tank you are already doing what you can to minimise the damage you take, this should involve holy shield, shield block, sacred shield etc. etc.
My position is that popping your shield wall at the start of the fight as someone previously suggested, is not the smartest thing to do, as it wont be available for when you really need it.
Right, you should always make an informed decision about Shield Wall and other cooldowns based on the fight mechanics. If you know there aren't any specific damage spikes, it might make sense to pop Shield Wall early and then later, but often you know when it's needed and should use it then. My point was that on that fight, you're going to get the bleed on you more than once, unless you have amazing dps. A lot of tanks forget that their minor cooldowns are significant too, and forget to use them. I'm just saying that because "the type of damage that kills you" isn't blockable doesn't mean that Shield Block won't help you. Not arguing with you here, just adding.
For certain fights, such as onyxia, where the boss doesnt actually really "do much" (except a steady stream of hard, but very survivable hits) I dont have any problem with popping the shield wall CD very early. Infact in fights like onyxia it could be a good idea to pop shield wall while doing the pull to make its a bit easier to get in position. But yeah, its always about "knowing whats going on" if youre going to be able to plan the use of cooldowns..