Yeah the BPS system does seem to give 6 points to scoring defenders versus 4 for forwards, so it's possible to look at players who always put themselves in the box a little (Kompany/Koscielny both come to mind), also clean sheet bonuses for defenders mean if you look at the defensive records of top sides then it is going to give a higher average points to clean sheet teams which would make Mignolet a top value keeper. This also makes possession teams more valuable, since drawing 0-0 still gives you defenders a good amount of value.
I actually thought BPS applies to every player, but I do see what you mean by it's of limited value.
I'll play around with it a bit and see if I can do any better