There seems to be no shortage of players here who prefer old, dead strings. But I'm not one of 'em. And I'm certainly no fan of dirty strings. Yuck!
I guess I could see getting a set of flatwounds and keeping 'em on the instrument for a long time - if you're really into that "maximum thud, minimum tone" kind of thing. But even then, it seems to me you'd at least want to keep them clean...
When strings become very old and dirty, they're virtually impossible to keep properly tuned and intonated. They no longer resonate properly, so that the harmonics (oh that's right...you probably don't
care about harmonics) no longer ring out properly, so that you start to get weird "wolf" tones and/or dead spots, etc.
I just don't see how they would be useful for playing actual
music...
MM