A 12-pound spring doesn't necessarily translate into a 12 pound impact on the primer. The 12 pounds is measured when the spring is compressed. In the Ruger the hammer hits the transfer bar, that in turn hits the firing pin, and that then hits the primer. Some energy is lost in each step.
Ruger uses springs that are intended to ignite any primer under any circumstances. When you replace the springs for lighter ones you compromise to some degree this reliability. For some uses this might not matter, but in others it could be critical. Also if you are shooting rapidly in the double-action mode, and a light hit causes a hang-fire, you may set off two cartridges at ones. If or when this happens the revolver usually doesn't survive.