Partially this is because of guild identity and partially because of different rulesets within each guild's chat. I know I'll be spending time in GWOA, but that might end up being more when I'm logged in and not doing much rather than when I'm out-n-about being all mesmery. :)
Maybe it is just me holding onto an "old" way of doing things, but I really don't see the downside if ANet had put in an actual Alliance function with an alliance chat channel. Has there been any detail on why that was not done (other than the, "guilds can be bigger now!" statement)?