Time spent casting sleep is time that could be spent dpsing. I'm also fairly sure you can't build threat on stuff that's asleep even with Flash, but could probably test it more extensively. Reason enough for me to not use it unless we actually need to lower the incoming damage (aka, almost never).

Unless the healer is extremely undergeared and requests it (or if the healer requests CC in general to lighten their healing load I'll try to respect their request) or spends their extra time dpsing, as a tank I'll just wake stuff up if I have a THM/BLM spamming sleep when I didn't mark for it (can't remember the last time I did - Stone Vigil's 4 pull, probably).

If the tank is weak at multi-target threat than sure, sleep stuff as it's just gonna run around otherwise... but you won't really be using any damaging aoe moves with such a tank anyway.