I don't think it's useful to generalize about whether people 'should' 'stay and fight' or not - it depends so much on the individuals and on the circumstances, but anyway:

Ultra was able to successfully decrypt Enigma messages because the British evacuated Polish code breakers, so I suppose it's a good thing those people didn't stay and fight.

The Polish resistance saved tens of thousands of Jews from the Holocaust and gave a ton of information to the Allies, so I suppose it's a good thing those people did stay and fight.

Tens of thousands of Polish soldiers (under the direction of the Polish government in exile) fought with the Allies throughout the war, so I suppose it's a good those people left and fought.

Just some random thoughts, everything is complicated.