Agreed here.

hb/bs/ns spam does a little bit less than OB + proc HB on single target dummy. But It does more when you can hit more than 1 target which is like 99% of the case in arena. Your opponents can't be all perfectly spread out. Not to mention hb/bs/ns spam is so easy to fit into a 1 button spam.

With that said, I see OB is probably worth using though. Meaning just throw it into the rotation, if it goes off, good for you. If it doesn't, HB/NS goes off is ok too. When killing machine procs, it is better to OB than to FS. Taking OB talent doesn't make you lose much in the unholy tree.