Belts wear out disproportionately faster, as do bearings as spindle speed increases towards the max.
Don't know what the actual numbers are, but I know a 15,000rpm spindle will last a lot longer run at 12,000rpm than at the full 15. Even longer at 10,000.
The belt on our 10,000rpm mill is almost 8yrs old, and looks new. I usually keep it down to 8000, but mostly run at 5000 and lower. Service guys tell me that machines run at max will go through belts in 6-8 months.
A friend of mine reports that by dialing his machines back to 80% of max spindle speed is rebuilding his spindles less frequently, such that the loss in productivity is more than offset by the decrease in machine downtime and spindle costs. Again, numbers, I don't have 'em.
If you've got a big enough operation to justify keeping a spare spindle cartridge on hand, that'll at least soften the downtime issue.