You really can't hit your unsummon macro before minimizing/hiding the game?
Not everyone plays the game using macros, also some people do play in full-screen and just turn off their monitor/T.V.. When their going to take a break or go AFK. Having an Avatar out shouldn't be any different from when people AFK at the entrance to the Mog house.

I personally think that's more of a problem then having a Avatar out, makes the cities and areas around it so laggy. But I can see and I do understand why people do it and I'm not against it. It helps to see what /shouts going on cause you can't see them inside a Mog House, and that loading screen takes forever and a day to pop up... Just to zone into the Mog House so, in other words if you gotta go, you gotta go. To jello (yes I said jello,) on waiting for the loading screen.