Would a 64-bit client really make any difference?
I mean, isn't it so that the limit for a 32-bit program is it can use a maximum of 3GB memory EACH. A wow-client uses only about 1GB so there still is a long way before that limit will cause problems (unless Blizz prepares for a major game engine update).

Just as long as the OS itself is 64-bit all those clients can run in their own "32-bit bubble" being able to access maximum of 3GB each (if total amount of RAM is large enough)