Yes. Just looking at Socrethar log, you rended the shadowcaller (caster adds) zero times, which is BAD.
1. Rend does great damage for rage spent even though its damage doesn't benefit from mastery.
2. Rend generates rage with TFB if specced.
3. Rend procs MS.
If there's 2 or 3 targets you should be rending all of them unless they will die too quickly.
And yes, legendary ring is huge and much stronger than any of our CDs, so not having it is a big deal.