I think that's a good reason why one single global chat channel would be a just a bad idea. Too many conversations going on at once does not make for a conducive environment.
Global channels are not meant to be a replacement for the party seek feature as that's something that is, hands down, needed. But, I feel global channels can fill in the gaps when a party seek feature falls short.
Suppose, if you will, Joe Blow just pick up some leves in Ul'dah and while he can solo them all, Joe would prefer to get together a small party. Now, Joe could try the party seek feature and hope the people looking for a party wouldn't mind doing some leves for a bit or he could shout right next to the crystal and hope someone is actually there and willing to help. Or, perhaps Joe could send a message to the leve global chat channel that he's about to do leves at X, Y and Z and wants to see if anyone is interested in joining him.
Meanwhile, I'm out in the field hacking away at some botany nodes hoping for some sweet rare stuff, Teeny Tiny is in the market wards looking for a weapon that's not five times his height, and Miss Kitty is turning in some crafting leves at Black Brush ready to scratch someone's damn eyes out if she doesn't get a single guild mark points.
What do we have in common? While not actively interested in joining a SP party, we all belong to the global leve channel and are all interested in helping out Joe Blow with a few leves.
I'm just trying to highlight an example where people that haven't considered seeking could be pulled together just because someone was asking for assistance. My point being that one system supplements the other and is not meant to be a replacement.
And, of course, this is just an example related to party seeking. There are plenty of other uses for global chat and situations where it would augment features already in the game. (Or features that should be in the game.)