There's synergy and then there's tanking. Synergy wise, the Pally has more to offer because he can buff your party and regen mana for everyone. Tanking wise, the warrior can take more damage and has more "oh shit" tools at their disposal. But the number one reason MBers take pally over warrior is that it's a lot easier for a pally to maintain aggro on multiple targets with very little manipulation. Whereas the warrior needs a lot of (different) key spamming.