I have never noticed water dripping from my fender to the derailleur, unless riding through lots and lots of water. Usually, the spray from the tire runs along the underside of the fender and drips off the front and back, although I am certain that the RD is not immune to road spray even with fenders.

At any rate, without fenders you are spraying the same stuff directly onto the chain, derailleurs, chainrings, pedals, and cogs, thus introducing the same abrasives to the same components, but in much greater quantities with much higher and more forceful velocities.