Classes that stack strength or other stats can still get their overheat skills and normal attack boosted by Quicken. Chaplain for example goes SPR and quicken will boost their attack speed.
Stop prevents enemies from even being able to attack you completely, so it is a lot better than slow in ET. Floors where you have to protect an object or just stay alive don’t require you to kill monsters, so just stopping the monsters is effective. It also allows you to safely prepare your CC skills, most notably ice wall and snow rolling while enemies are stopped, and you can watch the Stop debuff countdown and make the best use of snow rolling/ice wall.
Also, you can turn off the attribute that allows you to slow bosses and use Stop to protect your party members from the additional mobs that bosses spawn, allowing them to more safely focus on DPSing the boss. If you just used slow, it wouldn’t be of much help in this situation.
For DPS check floors, the whole party just stays in a corner and fires off AoE skills, so slow is not helpful at all at helping your party members “reposition” either. Actually, if you slow enemies down while they’re heading towards your party, that might hinder your party in these cases since the fastest way to clear the room is to mob the enemies up in one place, and if they’re slowed they might take too long.