It amuses me when people drive "defensively"... as if championship points, sponsorship $ or career opportunities are at risk.
I don't "let people by" but i sure as heck don't hold anyone up on purpose either. I run the racing line that works best for me, if they are good enough to get by, so be it, if not, then so be that too.
I much rather try and race a spot back, then hold onto a spot by "defensive" driving. Defensive driving to me says, I'm not confident in my ability to race you back, so, I'll just be Mr. Jack-o-lantren. I feel bad for those people.