This could work on a group basis but never on a per person basis, The poor healer keeping the party alive who does only a few % of the total damage would only get a fraction of the SP (if they were lucky) a DPS would per kill.
A per person approach would effectively remove all healers from SP parties overnight as there is no incentive for them to party as they could get 50 times more SP per kill soloing.
It has a whole host of other more subtle problems about it too such as mobs that are immune to magic or physical attacks. Tanks would also favour mobs that are tank and spanked over mobs that are kited.