The DC pump assists in fuel pressure boost for APU startup when no AC power is available for the L main tank pumps - a consideration for the -500 which can operate out of smaller (and more bucolic) airports where power carts/ground AC theoritically wouldn't always be available.
The -500 is usually equipped with a dedicated APU battery using the same reasoning - self-reliance on the ground.
I wouldn't be surprised if a few airline-spec'd -500's did not have the DC pump - but don't have any direct knowledge regarding the omission.
EDIT: Oh hello Gary - simultaneous post nearly - you beat me to it
