People MGEing on alt's makes alot of sense too me. Sometimes you want to just be able to train while keeping your ego out of it (to echo what mikeikeike already posted).
I'm sure if there was a mod that had you play against bots that were comparable in skill/play to actual players alot of people who use alt's would just do that instead.
As far as people playing really annoyingly in MGE, I don't mind it to be honest. I just take what they are doing and use it to challenge myself, it's kinda like a harder difficulty. I'm not really concerned with winning when I MGE so playing against difficult/'gay' tactics doesn't really bother me, infact I enjoy it.