http://www.silkroadforums.com/viewtopic.php?f=2&t=99711 is an active thread discussing the same topic. The reinforce is a multiplier, as you can see by the formula below. Which is why most people prefer reinforce over weapon damage. However both are important if you want to deal the best damage possible.
Here is another which includes a pretty accurate formula:
http://www.silkroadforums.com/viewtopic.php?f=2&t=62990&st=0&sk=t&sd=a&hilit=damage+formula Quote:
phy__base_dmg = (weapon_phy_reinforce * str + weap_phy_dmg) * mastery_bonus
mag_base_dmg = weapon_mag_reinforce * int + weap_mag_dmg