Back when I first started leveling in vanilla WoW people actually didn't ask this question. Spec was something people only worried about for raids. I seriously didn't care if the warrior tanking for me was Arms, Fury, or Prot, and honestly it didn't matter.
The Burning Crusade was a bit of a strange animal though. For one thing, classes other than warriors began tanking in earnest, so those paladins and druids that formerly came along as healers or dps could possibly be tanks as well. For another, there were people emerging from a raid environment that expected people to spec for what they do in a party (even a 5-man) and that anything less would be a disservice.
My early levels in TBC I tended to advertise my druid as a balance/resto hybrid whenever someone asked. If they needed a tank I couldn't help them, but I was more than willing to heal, and I got into several pugs that way. By the time I hit 70 I was in a solid guild and pugs were no longer necessary because I typically ran with a guild group. I became full-time moonkin.
Now I'm leveling again in WotLK and some nights it seems the whispers don't stop.
"What's your spec?"
The funny thing is, I don't even know what answer they want. Sometimes they'll specifically ask if I'm feral, but they never ask if I'm resto, even if that's the spec they're actually looking for. And I know they're guys trying to pull me into an instance rather than asking me for spec advice.
Usually I'll just answer balance and let them respond with a wall of silence, but if I might be interested in pugging at the moment (because I consider it good practice to pug) I might say something along the lines of "balance, but willing and geared to heal."
Sometimes they take me, sometimes they don't, but we've finished every run I've healed in Northrend. Which brings me to a couple things I've always believed about playing a hybrid in an instance:
1) Leveling dungeons aren't intended to be an exercise in min/maxing where it's necessary or even strongly desired for people to spec for their role in the party.
2) Any spec should be able to perform any role assuming they are properly geared. Underline the properly geared part.
One of my shaman guildies was kicked from a Azjol-Nerub pug because he wouldn't respec to resto for them. Note that this was not a heroic or even a level 80 instance! So he and another guildie (who was kicked along with him) formed a new pug and they ran the instance in about 20 minutes, everyone at level, and with an enhance shaman healing. Common sense is seriously the most important thing that's needed for a leveling instance.
Spec can make things easier for the group if everything goes to pot, but it's desirable for most groups to avoid going to pot in the first place, and many healers find it easier to level as feral, enhance, shadow, ret, etc. Just because their spec changed doesn't mean they forgot how to heal.
I'm much more likely to answer positively to "Can you heal this instance?" than "Are you resto?" or worse "What's your spec?"
Obviously this changes with heroics and raids, and I don't blame the spec question then, but for leveling instances? Pfft...
The thing I like about hybrids is that there are other options available if my main spec doesn't cut it or isn't immediately necessary. On my druid I can be dps or heals. On my paladin I can be heals or tank. Two of the three party roles I will be available and prepared for. And if really necessary I can even try the third role.
I was trying to solo The Plume of Alystros and found I was dying as a moonkin because I couldn't root the darn bird. So I decided to get creative and try to bear form the bird down. I put on all my tanking gear, buffed myself, and got ready to summon him again.
But before I could, a priest rode up, saw the quest I was about to start, and invited me to his group. A warlock rode up a split second later and was also invited. So here was this balance druid completely dressed up like a tank and a group of three people, two of which were clothies and it was quite obvious neither of them were going to tank for me, and unless they looked at my spec, there's no way they would know I was balance. Every piece of my feral gear was TBC epic or Northrend blue.
So I decided to make the most of it and tanked that bird. It was not the cleanest fight, since I wasn't used to bear tanking, and I was squishier than I'd hoped to be, but even as a balance druid with no points in feral my threat was incredible! It was much better than when I tanked Ring of Blood back in Nagrand. With some more work on my mitigation I think it may be possible for me to bear tank an instance as balance.
And that would be an interesting to reply to "What's your spec?"
"I'm balance, but I could tank or heal if you need me to."