Meaning: Someone with good values, not afraid to be politically incorrect and does the right thing, and in general on our side in this fight

Proper Usage: The based professor who refused to grade students on a racial-curve got fired.

why the hell every lemming jumps off the same cliff?

I dont think people using this term correctly is the same as all lemmings jumping off the same cliff.

Is it simply cuz you saw someone else use the term so now you have to be cool too?

No, it does not make you cool to use the word "based". However it does express what you want to convey very succinctly and is widely understood, especially on our side of this War.