I didn't even know you COULD use shift-click to use alien abilities.
I myself prefer the MMB, simply due to ease of access instead of having to hold down another key just to do an ability in the blink of an eye. A lot can happen, yuh don't got time to fiddle about with the keyboard, MMB nice and quick!
Moreso when you are an alien like a ravager or a sentinel, let alone a queen.