What I found was that when my tires had 5,000 miles on them, they didn't look so bad to me....until I bought a new set in preparation and compared the old and new tires side by side. Oh my gosh, it was scary how worn they actually were without my 'seeing' that!
The thing is, I'm a pretty safety conscious person, and what I really want to avoid is having a tire BLOW OUT while I'm going 30mph on a county highway here with no shoulder with a big truck passing me at that moment. My worst nightmare.

So now I have vowed to change tires a little more frequently. I never want to leave them til 5,000 miles again. The hell with saving $10 by getting a couple more months out of an aging tire.

I think another factor in tire wear is how smooth the roads or paths are that you usually ride on, and how much debris you deal with on a daily basis.