Flesh Rippers equips on both hands, that mean you cant use them with shield at the same time. The damage scales with Fist level yes.

The Choice is yours, the set dont have that much armor at level 4 (48), but that extra 9% ms might save your ass once. Having a good shield and a 2 set of HP food might be the best solution to that problem. So im gonna say yes, its worth it to level