well.. it automates decisions for you.
'Smart aoe", things like that.
But honestly, you can logically do the same things without that, just use a custom toggle, and make a macro ingame for that toggle (or click it) theres times when you DONT want it to cleave, yet the smart cleave will.
Example, HM Imp, the little aberations, you dont want to burst them all down instantly, or death will ensure, if you have smart AoE, itll just cleave for you. Where as with a toggle, you can use a keybind to toggle on/off if you want to cleave or not.
Boom, just saved a monthly fee!
Plus I like to have more control over my rotation.
and before you say you can toggle smart cleave on/off.... whats the point then, your still using a toggle, might as well stick with a non-smart cleave on/off, functions the same without a monthly cost