actually tribulus does increase libido and erections, but not because it stimulates LH or testosterone production. Those effects are related to dopaminergic, NO and oestrogenic effects.
tribulus is not useless, its just does not do what it claims (raise LH and testosterone)- though it has effects that would lead one to beleive (incorrectly) that it does.
just as a note- there are no studies that say that PCT is useless. In fact there really are no "PCT" studies per se. There studies on HPGA restoration and fertility, some of which are case studies of anabolic steroid users (typically after continuous significant shutdown- as opposed to post cycle studies).