I bash whenever possible with my warrior even though I give up my offhand weapon for about 1-2 seconds with a bandolier macro.
The chance to stun a mob for a short time means I take less damage and it greatly increases aggro.
Plus, the offhand doesn't have the same kind of damage bonus as the main hand, so I don't feel like I'm losing too much DPS.
Even though I'm a large race that gets Slam, Bashing with a shield seems to me to stun more often and definately does a lot more damage. To answer the OP, sorry I'm not sure of the exact relationship between the shield and bash damage, but I bet bash skill has a lot to do with it.
Edited, Mon Nov 7 11:51:22 2005 by JoltinJoe