My first cloak was on a tank, and I would agree it really wasn't needed. The extra death-prevention is definitely nice, but it's so uncontrollable that I wouldn't merit it needed. I haven't gotten a Healer cloak, but I would probably say that same about it. It definitely is more useful than the tanking one for heroic bosses where extra healing is always welcome.
As far as the DPS cloak, it's absolutely needed for progression, if you're still progressing. The meta and the cloak are HUGE dps increases, you simply will not compete without one. I just recently changed my main from Prot Warrior to Hunter and I joined a guild where most had cloaks, and I was still on the Valor part. I came un undergeared as well, but after a few weeks I was right on par, gear wise, but was still only on Runestones. The Meta jumped by DPS by a bunch, and once I got the Cloak itself I was #1 or 2 most fights. Moar dps = quicker boss fights = win.
Obviously if you're just doing LFR/Flex you don't care about progression anyway, so it's not needed there for sure. (Unless you are super competitive and always want to top charts)