I have experience the first half of Ulduar 10-man (excluding The Assembly of Iron) and notice one thing, a lot of adds appear during the boss fights. Is this Blizzard subtle way of saying, you may want a paladin to offtank the adds. Then your fight will be easier.
Think about it. Numerous adds will appear during the boss fight and must be killed asap. A paladin has better abilities to quickly hold the adds at bay while the DPS kills when no issues of anyone pulling aggro.
For example: Razorscale has adds appear when the boss in-flight. If a paladin offtanks all of the adds, it allows the DPS to focus on one tank while the maintank can worry about the Sentinel (if they appear) and the boss.
I am not trying to discourage groups not to use other tanks (Warrior, DK and Druids) but if an encounter is difficult and the adds are too much a paladin is your best choice to AoE Tank.