It's difficult to tell just how thin the tread on your tires is until you replace them and observe the difference. I finally replaced the tires on my Segway last night, and the difference is very obvious if you see the before-and-after. I was planning on just waiting until I punctured one, so I'd get a little extra life out of the existing tires. But, a few weeks ago, I figured that it's been two or three years, and I've probably gotten enough life out of them, and changing them now means I have spares in case of emergencies.
It's hard—for me, at least—to justify replacing something that appears to be functioning perfectly well. It's a little easier with things like air filters, since you can see those getting clogged up and black, but with a tire it's hard to see the difference, and nearly impossible to feel any difference, so I feel like I'm spending money on nothing at all, and that doesn't sit well.