Originally Posted by Bolvar

It's not D&D-style multi-classing, but the idea is born from the appearance of NPC's in-game that have obvious traits from two classes. The two-class requirement is really just a gating mechanism, similar to the level 55 Death Knight requirement, with the hope that leveling two similar classes will make the transition to the accompanying hero class a little smoother, as the in-game mechanics should bear some familiarity.

And, obviously, as others have so aptly suggested, it works for more than just the four examples I gave - those are just the ones that came to mind, and fit what I thought was a need to add more tanks and healers, as well as maybe give a few more classes reasons to roll on obscure armor like spell chain and plate.