I found rend to be a relatively useless skill while training especially at lower levels because one does not have the right gear and points into strength to make it effective. While it's as fast as an auto on a haste, the DOT is pathetic without a spear/DL dagger and above 500 strength.Isn't it sorta bad that in 15 sec a skill only does the equivalent of one auto?
Idk what youre smoking but i loved rend when i was leveling, and love it even more on emdgame bosses
However, it is a valuable skill at high-end status as you have already stated.
My recommendation is to drop it, and go for a dex build all the way to level 190 if you are training especially on wyrms at higher levels because you want to minimize restoration pot usage and deaths. Invest points into fast reflexes, life steal, riposte, quick strike and shadowstrike and you will see the wonders of a dex build.