Okay, if you took that wrong, I'm sorry. I have a shaman myself, and know that we can "tank" (as much as anyone tanks) in lower level instances. I've done as much in Gnomer and SM. But anything high level (particularly any raid instance) a shaman would get eaten alive trying to tank or off-tank.
Shamans/Paladins are the 2 role hybrids. Shamans heal/DPS and paladins heal/tank. The only class that's really capable of filling all 3 roles is a druid, and that requires being in any of their myriad of forms. I guess my point was that you can't really expect any class to be able to easily fill more than 2 roles (druids require a lot of finagling and put themselves in not wonderful situations trying to switch between roles in the midst of a fight) because that would, in its own way, overpower the class. It shouldn't be possible to do any high-level instances (5-mans, heroics, or even 25-mans) with all one class type (again, with the exception of druids, what with the shape-shifting), or other classes would have fewer chances to get into groups. After all, why take someone who can only do one thing when you have someone who can seamlessly shift between whatever needs to be done without missing a beat?
It's just a matter of balance that shamans can't tank, and I think that it's a good one, but what do I know?