This post is inspired by a thread on the WoW forums. Unfortunately, I can't find that thread anymore. I really should start bookmarking good threads.
Imagine you have a DPS warrior in a lower level instance. Suddenly the tank goes down. What do you do?
You swap in a shield, taunt, and take over tanking. You probably aren't as good as the tank, but you can substitute well enough to finish the fight.
In this case, you are acting as a "fluid" hybrid, able to switch from one role to another while in the same fight.
The other extreme type of hybrid is a "modal" hybrid. A modal hybrid can switch from one role to the other, but not in the same fight.
A lot of people roll hybrids because they want to be fluid hybrids. To be able to equip a shield and take up tanking if the tank gets into trouble. To be able to drop back and support the healers. But, increasingly WoW hybrids are becoming modal hybrids. Your paladin can be a healer for this fight, or a tank, but you have to choose before the fight starts.
Indeed, the fluid hybrid disappears totally at endgame. All that's left is modal hybrids.
I think this theory goes to the heart of my current dissatisfaction with the paladin class. I rolled and played as a fluid hybrid, especially in 5-mans. But now, hybrids are almost entirely modal. For this instance run, I am a pure tank. For this boss fight, I am a pure healer.
The interesting question becomes why have hybrids in WoW gone almost entirely over the modal side?