Well in dental school we had to buy our own burs at $2-$10/bur for a
student that got expensive, patient's paid the school but we were not
reimbursed for the burs. So we used the burs until they were dull or
broke. Only for an exam we would use a brand new bur. What else do you
expect from a poor student?
At the same time I have never seen a crack line develop from using a
dull bur. And diamond burs are different because diamonds don't dull
per say the diamond coating separates from the steel shank, or if the
diamond is used without water it burns to charcoal. So basically the
diamond takes layers down without creating crack lines. If something
breaks off as you are cutting it, usually it already had a crack and
was meant to break off anyway.

They just didn't teach us any better and we were poor (not wealthy) students.

I quit using 556, 557, 1556, and 1558 burs when I noticed micro-fracturing
shortly after using 2.5X magnification.

Now routinely use 331 and 331L for most preps, yep even for endo access.
Tried 245's for a while, didn't really like them,  but they didn't cause micro-fractures.

A fresh bur always cuts better ;-)