Besides all the obvious reasons like ISO and going wide I will switch to Full Frame because we all know that FX is here to stay. DX is here for at least a while but it's very unsure if it will still exist in about 5 or 10 years. And if I buy (expensive) glass, I want to buy it for what it is and will stay. If you have a DX camera, you may need a 35 mm and for FX you need 50mm for the same purpose. If you eventually switch to FX when DX stops you are left with a whole different set of glass than it was when you bought it. And if you want to go DX again, you can always buy a converter.
Makes me think it would be pretty smart if Nikon could make a converter the other way around, like a 0.7 converter to use DX glass on FX bodies. But that's a whole different story.