If there were any heal AM you would be right but as it is...? BW procs arent really worth it, their impact is minimal nowadays. The GCD is better spent maintaining for example magical damage reduction with BO if that is a problem. SW's VON is a decent enchantment, true. SM's WW is good on paper but very few classes actually utilize it well and then its easier countered with a CD increaser.
In regards to your edit: Complaining that Rampage+Shatter Limbs is capable of disrupting the seamless use of HTL is kind of hypocritical considering that HTL itself is a borderline broken ability.
The main thing I see with the reduction is that Rampage wouldn't simply ignore morales. Which would be a good IMO but could also be fixed on the morales side.
It applies in small scale for sure. If rampage were such a big deal any BO could get the CD decreaser and spam shatter on the slayer until its gone... through all slayers of protection. I think nobody does that though because there is no need to. Even without you have multiple chances of shattering it before slayer starts to do serious damage (lets be real, Tesq virtually nobody plays with rage drop tactic because it a pain in the ass)