Okay, I'm not trying to be deliberately obtuse here, but wouldn't a thorn or nail puncture always be big enough to let in debris as you ride, assuming the thorn/nail went all the way through the tire and punctured the tube?

I guess I'm not seeing the distinction. I have exactly the same issue as the OP. I have a tiny hole in my tire from a sheet metal screw that was on the road a few months ago. The screw went all the way through and popped the tube - the thing was lodged in there like a thumbtack in a cork board. I put a folded dollar bill on the inside of the tire over the hole, installed a new tube, and went on my merry way; haven't had a single flat tire since. But lately the tire has been losing air. After three days, it's flat (less than 20psi - this is a 90-110psi road tire). I assume that the original nail hole finally rolled over exactly on top of a wee sharp rock or debris, and this has resulted finally in the tube getting punctured in a tiny way, and letting out air very gradually. Or not. I'm not sure. I'm not a tube physics expert. Would it pop, or would it leak slowly, if this were the case?

Sheldon Brown et al say you don't need to patch nail/thorn holes, but I don't see how this would be possible. Wouldn't any hole that goes all the way through the tire be vulnerable to a tiny rock making its way through the rubber and puncturing the tube eventually?

I'm trying to decide whether I should buy another tire, or be a good ecological citizen and use my existing tire for as long as possible.

By the way, if anybody knows where to buy Schwalbe Marathon Plus tires in the Silicon Valley area, I'd be eternally grateful. Mail order is great, but takes so long.