(Obviously, I stole the line from Jessica Rabbit...)
Pre-BC, hybrid classes could do just about any of their roles without having to be specially designed. A shadow priest, with appropriate gear, could heal. A retadin could, with a sword and board, tank. And yes, they obviously won't be as good as people who are designed to do those roles, but they can get the job done decently and adequately. Even well, if they're a good player and have appropriate gear. (ie: don't try to bear tank in cloth healing gear)
Once you hit the Burning Crusade and Outlands, it becomes a slightly different story. You start getting to where you're expected to be the spec that you are impersonating. Can you still switch-hit? Yes, sure. And probably still do a good job. But you start to get to where you need to have the special abilities and spells that deep in particular trees may give you.
Hybrid Class roles that overlap:
There are a few hybrid class roles that can overlap (with the caveat that the secondary role is not as good as someone whose primary role it is) and they can overlap well with good players.
Boon, the healadin, can become a tankadin. His paladin is specced such that with a simple change of gear, and being the awesome player that he is, he can main tank the 70 instances (regular). He's even OT'd in Karazhan leading up to Curator when we had one of our tanks step out to allow more DPS in on the Curator fight. He can hold aggro well, provided that the DPS classes understand that he is not a tankadin and thus does not have Avenger's Shield and some of the nifty abilities that help them generate threat. He has to build threat slightly different, but he does it well.
However, he doesn't delude himself into thinking he can main tank a heroic, or main tank Karazhan, or even OT Karazhan for more than a single pull here or there. And when people say 'Hey, we need a tank, Boon, come tank for us!'... if it's a heroic, he says "Are you nuts?"
Feral Druids can be either melee DPS or tanks. This involves mainly a gear change, and maybe 3 points swapped about, but with little other need for changing their talents around, they can do both respectably.
I'm sure there are others -- Chickens masquerading as Deleafed Trees, Lightning Casting space goats pretending to be Healing Wave surfers. The question is... can they pull it off? Not merely the spec that they don't have, but the gear, the ability, the understanding?
I think they can, if they have the ability to understand what they lack and how to make up for what they lack. No Earth Shield? How do you get around that? No Swiftmend? How do you get around that?
And most importantly, if you're playing 'offspec', let the other members in the group know it. Don't surprise them one pull from the final boss, after an instance rife with 'damn, I died again.' moments, say 'I'm specced elemental.' (which I suppose is at least better than 'I'm specced enhancement.')