Really, it comes down to carbonation volumes. If you have a dual regulator, then you can carbonate two beers at difference pressures. This helps if you have, say...a belgian, and an english beer on tap at the same time. It also would let you force carbonate one with the 30psi method and still have one on tap.
Personally, I think the 30psi overnight carbonation method is junk. I also think that most people (regardless of whether they think they can or not) can't tell the difference between pressures when it's a 1/2 a volume off. Because of this...I've always used distributors, and I'll continue to do so. I carbonate all of my beers at 12-15psi depending on the temp of my fridge. Who cares about accurate carbonation anyways? It's homebrew.