The thing, I find, is that system mechanics explain, entail and give insight into the key premise of the game. If the game is about using high-powered, giant robots, the system must be focused on providing these rules in a fun an entertaining way. If the game lacks the mechanic that suits the specific "soul" of the setting, then playing that game in particular as opposed to another seems a bit futile. Note that I say a "bit" futile, because imagination is capable of expanding on whatever the system did not provide.