Only in some (read: insane) datacenter designs. You don't need spanned layer 2 domains unless you're doing crazy things like long-distance Vmotion. There's almost always a better way that invites less pain.

Totally agree, but there are protocols in place to tunnel layer 2 domains to encompass multiple data centers. VMotion between DCs is not as crazy as it sounds, it's just prohibitively expensive.