Unlike humans who need an armory, aliens can evolve just about anywhere. There's just one limitation - you can't be too close to enemies, which is perhaps ok to keep things sane (dretch next to you evolves to dragoon). However, it's very annoying when humans are attacking alien base and you just can't evolve. Humans could use armoury in analogous situation.
I suggest that it should be changed somehow. Either restrict alien evolving to creep (this would be consistent with 1.2 notion of making aliens more base dependant), or keep things as they are now with one small change. Aliens next to important structures like Overmind can evolve at all times, even with humans around.
Ban on evolving is very annoying on maps where you just can't move away sometimes, like atcs.