Now, we use a Warrior (me) and a pally as our tanks. Neither of us would have been able to use the items as they drop us below the boss crit cap.
Does anyone know why we should/how we can use these items? Are the item developers out of it? The only class who doesn't need defense to tank is the druid, and they either can't use the items or would waste item points on parry, making them useless.
These items are along similar lines as Praetorian's Legguards - Item - World of Warcraft from BT and are designed around tanks already being well over the Defense cap and subsequently defense being a bit of a wasted stat (this has changed slightly with wrath). The intention with these items isn't to use all of them at once, they are designed in such a way that they give you more of other desireable stats, whether it be armour, expertise, dodge or hit they offer more of them then similarly itemisted pieces with defense on them.
For example with those legs you get a butt load of Dodge, Parry AND expertise along with a socket to top it all off. No other piece thus far provides that much avoidance and expertise, however you sacrifice Defense to get there. You need to remember that these pieces are designed to be used with Ulduar gear, which subsequently has more defense than the previous tier.
These items are all from the 10man version. So far I haven't seen all the plate tank loot in there but from what I have seen, defense isn't overflowing, and definitely not enough to maintain uncrittable.
I admit I do like the fact that they have items that increase their value the better your other equipment gets, but having to wait (which for me seems like it will be a while before I could use) for gear that will probably have to come from the hard modes/25 man version seems like a waste for a 10man guild.
I agree with the other posters. I use the legs you have linked in an unholy avoidance set that features Repelling Charge. It's basically a way to juggle some of your gear to specialize into a certain stat (in the case of the legs avoidance while maintaining expertise for threat). If nothing else, it makes tank itemization more interesting o.O