If D2 is to be considered an mmo of sorts, then this should be a feature. It would be helpful and convenient to be able to be in 2 or 3 clans, and just switch which clan you're repping for at any given moment and I'd bet it would garner further clan interaction for a lot of people.
edit----
everyone crapping on this has clearly never played an actual mmo. Being able to jump around between a few clans is a beneficial aspect that every actual mmo has.
ediot 2--------
I love my clan, but I've also come to really like a group I've been raiding with that is another clan. I've been invited to join the other clan, but I wont do it because of my clan i've been in since D1....but I'd love the ability to switch at will, because this particular raid group is getting along really well. It's literally 5 of a clan, and me in another clan. If that makes sense..