I don't know about other classes, but my shaman has an AA slow that does not require a spell slot. He also has a AA debuff. Both have fast refresh timers, there is no need to mem either as a spell. A 75% slow is more useful than a 50% slow. On a mob that has no slow mitigation (I know there are few of those these days, but it is the baseline) it means that you tank will take half the damage with a real slow on the mob. The difference is smaller with mitigation, but that still equates to a lot of hp in current content. That in turn impacts the amount of healing required, which in turn affects the amount of mana available to your healer and the mana usage rate. That means the healer can do other things, such as debuff/buff/nuke/root/cure or whatever more effectively. The fact that the tank is procing a weaker intermediate slow also means that the primary slower has time to do other things on the pull first, such as debuffing, assisting in CC, patch healing etc. I am sorry that you don't see the utility in tools that make your job easier and your toon more versatile, but I do and I welcome them.