Maybe it's the "security blanket" effect, the same reason we wear shoes with soles with holes from walking a million miles in them, or go to sleep in old t-shirts soft enough to clean the car with, or wear old cotton briefs with elastic that have since disintegrated. Chances are our mothers/girlfriends/wives have tried throwing them away and even replaced them with new ones, but we move heaven and earth to retrieve them from the dumpster. Even if the instrument "seasoned" under somebody else's watch, it somehow feels or sounds better in our hands because the sharp edges (in more ways than one) have since become smooth and become "acceptable" or "at home" to our senses, whether it be the sense of feel or hearing. Others call it "Mojo". Baka "hiyang" lang iyan *cringes when the old Kris Aquino TV commercial comes to mind*.
While typing this, i thought of something: maybe it's a "guy" thing (and Ladies, I don't mean this in a sexist way)! If grunge was the latest fad, men would wear worn out shoes, jeans with holes, play battered guitars, etc. because we felt "at home" with them. Of course, we told the world that doing so was our way of being "anti-establishment", but it really boils down to just being "at home" with them (amineeen!!!!). Just looking at photos of musicians on the web, I noticed that if you find a pic of an old, worn-out, even battered guitar/bass, chances are it's slung on the neck of a guy. I'm not saying you won't find women musicians who use relics, but from what I've seen you'll find more old and seasoned guitars in the hands of guys than galls. Am I making sense?