It's a bad idea because you are actually able to gear up your alt in perhaps full Ilvl 353 / 346 without even playing on it. It's the very reason why Blizzard is against class changes as far as I am aware, because you technically have a fully geared character who should at least be able to play decent but hasn't ever played. It's just odd.
On the other hand, now we get people who are totally clueless in shit gear or even better people who actually obtained a fully epic geared character themselves and still have no idea how to play their class. And it could fix queue times even more, because people definitely (at least I would) would be using their tank character to deck out their dps alts.
My conclusion however, remains the same as I stated at first. I don't this idea is awesome at all, it will probably cause more trouble than good.