DC can't have PM ability, therefore player with PM on DC position doesn't use the ability at all.
I'm not 100% sure, but I think there is no such thing as effectiveness of SAs, they either work or not. Maybe DMC/L/R has less chances to use PM than MC/L/R (if you meant that I tend to agree), but I believe there is no penalty. The logic is binary here, otherwise it would be unnecessary overcomplicated. I think =]