You should not deny picking up dual spec, if you do you're cutting your usefulness in half.
You will get more gear having a tank main, because there is considerably less competition for tanks per raid. Typically only 1-4 main spec tanks in each raid, depending on the fight.
I am a mainspec tank, and despite being very well geared, I'm usually dpsing in my offspec. On the occasion that one of our other tanks doesn't show up, or we need an extra tank, it is very nice to be able swap back and forth to fill the gap.
My biggest obstacle to getting my ret spec geared properly is items with no armor class. Necks, rings, trinkets, cloaks. There are few plate dps wearers(in comparison to all the other dps classes that are out there) but these items are hardest to come by because you'll be competing with all other melee for them, regardless of armor class. Sometimes it's obvious that a particular item, while it might be an upgrade for you, is better off suited in another's hands. For instance, anything with agility instead of strength, those pieces may be an upgrade, some certainly are for me, but they're much better suited for a rogue or hunter, so you should do your best to realize these situations and pass accordingly. Anything that has armor penetration on it - much better suited for a dps class that isn't a paladin.
That being said - I have a few pieces of gear in my ret set that violate what I've just said above, but they were upgrades from what I had, and I was uncontested on the roll.
You will get far more play time as a tank, but having both specs and gear available to perform equally well in both specs, will ensure that you hardly ever have to sit out of a raid due to class balance. Being able to perform dps and tank, they'll likely need you to fill one of those roles, and being a pally you bring valuable buffs they can't get elsewhere, while also being able to pivot between two vital roles to raid success.
I have not sat out of a single raid that I haven't either been late for, or volunteered to sit out of, although this is highly dependent on your guilds needs.