They gave Shaman and Druids Superior Healing pre-50 (I think classically it was a lot higher level if at all) so the only thing you're missing out on is CHeal which is mostly reserved for charm pets. Any healer can single heal a group effectively if they're using proper CC. Just so you know most TLP's aren't pulling one or two mobs at a time, even with SKs/monks as puller. They're bringing like 5 at a time and just hoping the Enchanter can handle it. I blame WoW, but it does make for faster exp. Problem is if your comp is good enough you'll run out of pullable mobs for 3-4 minutes at a time.
GEBS are BIS until deep Velious. Kind of ridiculous when you think about it. Only INT casters and a few WIS races will be able to cap it before then and not need them. But generally that 9 int/wis gives you an extra slot to get a big mana item like a ring or earring or something instead.
Clerics are still wanted in most groups, but one of the most easily boxed characters. Especially with CH pets. Usually people play the ENC and the Cleric is either boxed by the Enc (which can lead to some wipes b/c that's too much responsbility for bad players) or some one else as their only duty at high level is to CH the pet every 5 pulls. Unfortunately groups don't tend to run druid/shaman as well as Cleric so there's not much of an offhealer role. Weirdly they run druid/shaman in groups and usually one heals and the other DOTs.
Tanks are definitely the most sought after for groups. Usually because they're the least fun to play, smallest utility kit (65% of tanks are warriors in classic even though they suck), always have to pay attention, and worst of all are at the healer's mercy. If you get a "smoke breaker" or "BRB kids" healer, you're dead mate. That's why tanks should go SK so they can at least try and FD. Unless you're an Ogre in Lguk, in which case you're too small to stand and are ducking and can't cast spells while ducking. Sorry.