I like Alltalk for events. However, Alltalk...
...encourages voice chat spam and
...makes the most vital team communication medium useless.
I'm a touch typer with a top speed of 130 WPS in small bursts. However, even that speed cannot best "tap Q and yell 'Spy is Auto' whilst not standing still and being shot."
If you want comradre, want to share jokes, or whatever, text chat is perfectly fine. I text chat jokes and jibes at the other team when in spawn, or waiting for metal to crank out of a dispenser, or whatever. But when the spy is sapping, and what he is sapping is my sentry, I want to be able to alert my teammates without having to shout over yabber on all talk.
Or, put another way, when I am a spy, and I'm sapping, and what I'm sapping is the enemy sentry, I want to be able to tell my team it's safe for the next 10 seconds without waiting 10 seconds for the line to clear up.
Single-team talk doesn't eliminate this, and it can be fun to goof off on talk (especially with HWG in the house!) but with that said, as the team has a stake in urgent communication, they're likely to release the talk key when they hear "spy" or "demo" or "help" in the garble.