I've come across this dilemma with Bellbell time and time again. I levelled her to be Ret. But she is, most definitely, Holy.
The problem started as soon as I got to 80. It was just easier to get groups as a Holy Paladin, so that's what I did. It was easier to get raids started if I chose to heal, it was easier to be involved in anything. So, in order to get things done, I continually switched to her Holy spec.
Well, common loot rules being what they are, your main spec is what you're raiding as, and those upgrades for the DPS set take a while to accumulate. It eventually came to pass that while her Holy set was nearly all ToC 10 and 25 gear, she was still wearing a blue hit trinket and Ebon Blade reputation boots, and a tournament badge sword.
I finally bought her two pieces of badge-only T9 ret gear...and she's never had an occasion to use it. In fact, it's almost like clockwork; as soon as I equip her Ret gear and switch to her Ret spec, a friend needs a healer. It's come to the point where I only play her to help other people (thus the lack of Bellbell-flavored posts).
Now, another Paladin in my guild has the exact opposite problem. He's been running only as Ret because that's the only gear set he has (he apparently didn't do what I did and save up every random piece of offset gear while levelling), but all he wants to do is tank or heal. So while he's sitting with the sword off of Onyxia bemoaning his lack of a healing trinket, I'm silently sending envy waves at him.
It seems like dual spec didn't really change anything for me, besides not having to pay for Bellwether's respecs to PvP. It made Bellbell's situation worse, because eventually to do the content I wanted to do, I had to play her "secondary" spec. I don't have the time to continually farm for her to keep her DPS spec up to snuff, and the fact that healers are always in demand and plate DPS is everywhere, there's just nowhere for her to fit in.
For example, the ToC 25 I ran on Saturday? Along the lines of three or four ret paladins, two warriors, and three or four DKs. Completely detrimental to the group for me to say "Screw healing, I'm going to overload our melee even more."
Well, who cares? Tell one of the other Paladins to switch and get your Ret on.
For one, I'm not a jerk. I'm not going to say "one of you respec just for me so I can bring in my eclectically geared ret paladin." The other paladins have tank offspecs, and we have plenty of (sometimes too many) tanks for my alt ToC 25 run.
For another, it gets groups together, and faster. I don't know how many times groups have fallen apart because we couldn't find a healer. Eventually, even if I'm in a PUG, I'll have to admit that I can switch to healing after we sit in LFG for half an hour with no bites. I don't have time to sit around in a LFG channel; with all the other things going on in my life, if I'm going to run something I need it to start quickly.
There's also healer guilt. "We wouldn't be sitting in LFG if I just switched to healing." "Time is wasting because I want to DPS." "They just need a healer...I suppose I can do it." "If I switch to my healer then maybe the tanks won't die."
It's not that I don't like healing. If I didn't enjoy it, I wouldn't have kept the spec. It's that it's not what my character was supposed to do. It was just for me to experiment with other types of healing. But I made a mistake at level 80, and even with the gear I have for Ret, if Bellbell ever sees Arthas, it will be wearing the wrong kind of dress.