[Parhelion]

I should have explained that more fully.

When someone has to go and open a fully separate client to chat, they do so because they have the very specific intent of chatting. While I can't speak for other players, for me, this makes me slightly more aware of my time spent between the chat client and the game client, because I am forced to literally choose which of the two I pay attention to.

When a channel is just a part of the game, tunable or not, it's extremely easy to ignore the content of the chat -- until something catches your eye. This is when channels become the most distracting, because then you'll be trying to juggle your responses between the chatter and the gameplay.

A separate client furthermore enables you to "leave" the chat (by switching windows to the game), finish what you're doing, and come back to still have it neat and orderly and (mostly) on-topic. This works vice-versa, as I can't tell you how many times I've lost my place in a game or forgot what I was doing because of a temporary explosion in chatter. If you're forced to tune out a channel, you're no longer able to be a part of it in most cases.

Essentially, it has to do with time management and subconscious attention, and how aware (or not) a player is.

Jaz's expansion on the slight psychological effects of even HAVING a channel at all really hits the nail on the head.