Kal, you are right under most circumstances. With the fact that a Warrior's "mana" bar fills during the fight while a Paladin's drains means that he will eventually pass you on threat as you stop using Consecrate or downrank to maintain a semblance of a mana pool.

However, when I tank I do my very best to NOT be restricted my normal circumstances : ]. I pretty much pay off my raid leader (if I am not leading) to put a Shadow Priest in my group. If I have a high DPS SP in my group, unlimited mana = unlimited threat.