Karthas has a post up about cat form Druids being neglected for raids.
We don't have this problem as much now as we would have. As I've said before, the changes to Expose Weakness (among other things) made Survival start to become mainstream in 2.1, and these days of course on the forum, all the cool kids are doing it. (Well, those of them who aren't BM, that is ;))
Even so, it can still be a bit of an issue, especially if you're more heavily trap spec. (Which I often have been)
As I wrote in my comment on Karthas' post, I accepted a boomkin Druid for an Underbog run once, which ended up being an awesome experience. As well as the trademark machinegun Moonfire, (which does enormous damage, albeit with rather terrible mana efficiency) we got the usual buff, as well as offhealing as well when the main healer was oom, which probably saved us from wiping on a couple of big pulls.
Personally when running a group, I'll take any offspec with a res (Ret Paladins, Shadow Priests, Ele Shamans, boomkin/cat Druids) since I like having as many members of a group being able to resurrect as possible; it adds a very powerful form of insurance against wipes.
Also, although they don't have any real range, an Elemental Shaman is utterly devastating within a radius of ten feet or so; an RL friend of mine with a main shaman was getting 2k+ chain lightning crits before 60. Then you've got the res and all their other usual good stuff; if as a Survival Hunter you've never played alongside a high level Grace of Air totem, it's an experience worth having, let me tell you. ;-) I had three or four shamans in my old guild, and I loved them.
Although probably not the first choice for some groups, the Warlock is also probably the single most overpopulated class in WoW from what I've seen, so you'll never have trouble finding one. Most people probably already know about their trademark, Soul Stone, but as well as that and their summoning for getting groups together, they can also give you elemental banishing, (VERY useful in the Steamvault in particular) AOE Hellfire, and the soulwell for major healthstones. While Voidwalkers/Felguards can't main tank the higher instances, they can temporarily seal a breach in a pinch as well.
Paladins; as long as you can trap around it, Consecrate tanking can be great. I've been with a couple of tanking Paladins in some Steamvault runs, and given the vast numbers of mobs in that instance, I'd actually take them in preference to a Warrior in that specific case. Nothing got past them. Defensively I've also always compared Paladins mentally with the cave troll that was fought during the first Lord of the Rings movie; facing them in Warsong Gulch used to be a lot like that. It's a degree of robustness that can be useful in groups. Then of course you've got the resurrect spell, as well as the awesome and much loved Blessing of Kings in some cases.
Shadow Priests; although a lot of people overlook these guys, that is unwise. The synergy you get with both a Shadow and Holy Priest in a five man is unbelievable; spot healing from the Holy, reinforced by HoT from Vampiric Embrace, with Vampiric Touch acting as a mana battery. Then on top of that you've got two people who are resurrect capable, for extra insurance against wipes.
In other words, if you're hanging around Shat and looking for two more for a group after you've got the holy trinity (tank, main heals, main dps) covered, and you see a hybrid or a Warlock looking lonely, call them over. If the player behind the character is competent, chances are good that you won't regret it.