Pures don't hate hybrids, they hate what blizzard has done to appease the whiney masses that play hybrids. Hybrids have their roles: They are there to fill a hole that can't be filled by a pure class. They're the gray area. They can perform multiple job roles relatively easily. The penalty for that versatility is they can't do any one role well.
That didn't sit well with them, so they wanted more. Shamans want to DPS like rogues and mages. Druids want to tank and heal like warriors and priests. Paladins want to heal like a priest, tank like a warrior, and dps like a rogue. Sorry, something has to give. The pures are being eclipsed by hybrids that can do 1 job extremely well, but at the same time provide a secondary ( and maybe even a tertiary ) role at diminished capacity.
When a druid can dps like a mage, pop a heal when needs be, and go into bear when he pulls aggro and survive until the tank gets aggro, why bring the mage? The mage has been turned into a vending machine.
When a ret pally can bring rogue dps ( and they will in 3.0 ), slap on a shield and tank if needs be, can buff, manage threat, etc. Why bring a rogue or dps warrior?
When a shaman can dps better than a mage, can be about equal to a rogue, and can heal better than a priest, and offer raid wide buffs at the same time. why bring a mage, rogue, or priest?
You see what i'm getting at. Hybrids have gained so much they can easily take the job of several other classes. In most cases they may not do them as efficiently, but in some they can do them better.