I don't mean 18 player only raid content but rather content that allows 18 to enter and boosts HP or number of monsters based on that.

Omen is a prime example of a good system.

The main reason is that less skilled players can do the content too, or less stress is put upon each person. It makes content far more inclusive and it makes it far easier to build groups from LFG.

Omen groups are made quite often via /yell, and most of the time the shout groups are alliances.