Having watched people do it both ways, I think there is a lot to be said for getting your skills down in a single tank setup before you switch to doubles. If you have really internalized what being horizontal looks and feels like, it's easier to get back to it when you add the confounding factors of head-heavy double tanks. The people I know who went to doubles before they had buoyancy, trim and position down in single tanks have taken much longer to get things right in the doubles.
Personally, I don't think you have to have a reason to dive doubles. I like them -- in the water -- a lot. They're very stable and are almost like cheating when it comes to buoyancy control. They give you a solid rock bottom reserve for any recreational dive. They don't have to be switched out between dives (although you have to have big enough ones to make the second dive a good one!) and the gas you didn't use on the first dive adds to the reserves you have for the second.
They are not useful if you can't manipulate your valves, and they aren't VERY useful if your buddies don't know how to use doubles or to help you if you have a leak.
They are very heavy and awkward, and many charter boats are not set up for them. You may run into problems securing them, or boats where they put the bottoms of the tanks in wells or troughs that make getting doubles OUT very difficult. (We had to go buy 2 x 4s to fill the wells on the Peace for our DYFO trip last year.) You will also pay a lot more for fills at most shops, since they will consider doubles as two tanks, even if you have only used a few hundred psi out of them. If you are also diving Nitrox, this adds up.
I still dive a single tank for a lot of my recreational diving, just because it makes life easier. I honestly think one of the dubious things about the DIR communities is that a lot of people end up thinking they SHOULD dive doubles when they don't actually need them and would, at least secretly, prefer not to. DIR works just fine in single tanks