I myself have seen these belts go as far as about 7000,8000miles myself. And even then, I've never had one on an 800 powered machine leave me stranded. I've had them start to chunk out, but never fully come apart until I got home. sometimes I'd be forced to run a belt for a week or two with a chunk out of it because I had no replacement and it would do it as long as it had to.
The 700 powered machine is a completely different story.
So for me, I never saw a good reason the change the $140 belt until it gave me a reason to. The thought of having the carry a new belt, and the tools to change it along with me for 800-1000 hours kinda stinks when I could use that space for anything else. In the age we live in with cell phones, 2 way radios and such, having one of them is a far better insurance plan for ANY breakdown vs packing an extra belt that covers only one.
That's my take on it.