Flat-wound strings are used by a lot of jazz players because of their 'pure' tone. There are very few additional harmonics produced, so the sound is almost flute-like. Usually the guitar's tone controls are turned down as well, so you're left with a a soft, blurred kind of sound.

What would make flat-wound strings appeal to EVH is that they sound 'dead' even when they're new. He wouldn't have to boil an old set in an attempt to avoid the break-in period involved in conditioning a new set. He'd lose the sound obtained by scraping the edge of the pick along the string, though.

Flat-wound strings are ideal for midi guitars, and for players who base their sound on effects, heavy distortion, and signal processing, rather than the sound intrinsic to the guitar itself.

Me, I've spent years seeking out guitars that sound good before I plug them in. A fresh set of medium gauge (D'Addario EXL 115W) strings brings out the tones that are hiding in those crafted pieces of wood.