Polybolos

[Dan Howard]

I am thinknig that the Bren gun being deliberately damaged is a myth. Its spread was considered too concentrated for suppressive fire but its accuracy was welcome in many other tactical aspects on the battlefield. I can't find anything to suggest that it was deliberately crippled to reduce its accuracy. Though I don't normally like Wikipedia this entry covers the main points fairly well.
http://en.wikipedia.org/wiki/Bren_gun

If the draw strength was too low to give penetration or decent range, then this seems the most likely reason for it not being used. Unless reliability was perhaps a problem.