It depends. If you're trying to save money, you can get by with a single regulator and just use a splitter to carb and serve multiple kegs. If money doesn't matter, it's nice having the additional regulator to dial in each keg to it's specific psi.
I use a single regulator in both of my kegerators, and the only reason I'm wanting a dual at this point is because I have a root beer on tap that needs 30psi and because of that, I can't use my other two taps.
If you don't intend on serving a lot of highly carbonated belgians or sodas, then there's no need for a dual.
The other reason a lot of people go with a dual is because they want to serve one keg and carbonate another keg with the 30psi method. This involves blasting the beer with 30psi for a day or two and expecting it to be perfectly carbonated. I think this method is a waste of time and only creates overcarbonated beer most of the time. If you use the set it and forget it method, then you don't need a dual regulator for carbonation.