Personally I like it being limited, so it has a more rewarding feeling, if used correctly. If they made it "toggleable", they would have to make it even less effective on ranged, in order to avoid players abusing it, despite that we already lose out on auto-attack, which is a big deal.
If people wants its CD reduced, than I would rather tie it to Disenage. For example every time we use disengage, the remaining cooldown on AotH is reduced by 15 seconds. That, combined with Born to Be Wild and Terms of Engagement could result in a really interesting gameplay. Also, every time you use Harpoon, the cd on Disenage is reduced by 5 secs.