yep, i agree... a stat like this would be pretty heavily influenced by the runners baserunning speed and skills and the opposing D specially since guess someone just gotta figure out a way to factor in sac flys etc. youre giving the bases gained by the guys on base equal importance.
just imagine the HUGE advantage a guy like Lowell would have over a guy like Liebrathal who hits behind Thome and Burrell.
not only that, but youre also relying heavily on the obp of the hitters hitting in front of the guy. so that stat could be heavily affected by the strength of the linup/ surrounding hitters.
anyways, i think bases gained by the hitter/pa is still a crude but a relatively accurate measure of how "effective" the hitters been which is what i think youre trying to calculate.
i think these kinda stats are very important and underrated.