Trash fights aren't that long, and it just reduces the amount of over-healing performed by spam heals.
It was some time ago, but I was once told by a tank that kills go faster if you dont slow since he gets to riposte more often.
It varies and depends on your tank and the healing and the dps of the group getting things dead. I can riposte everything I'm tanking in my frontal arc (and lack of slow means I get swung at more so more chance of riposte). If we're dealing with easy mobs, don't waste your time slowing, and just bring lots. The harder mobs get the more likely you want slows or CC and lower numbers of mobs because running back or rezzing back is not ideal.
Simply not true. While each hit you take carries a chance to riposte and riposte can be substantial in very limited situations (like swarming). It also carries an equally as likely chance you'll be stunned and lose a few combat rounds. I see it on my DPS output all the time with access to a slow proc, two short duration stuns, and one long duration stun. If I don't use the slow proc or at minimum the long duration stun, I get stunned just enough to stop my DPS output by 25%. Even a brief interrupt stun breaks a combat round in-progress potentially reducing hit/ability chances.
Why would you be stunned unless it's a spell? All tank classes have been immune to melee stuns from somewhere around level 70.