Why are stress fractures so bad?

Depends: Stress fractures are problematic mainly because they can be difficult to diagnose. As such, they may be initially under treated, thereby prolonging disability.

Not SO bad....but...: Depends on the location of the stress fracture. Just like the name tells, the "stress" must be reduced to prevent further damage. Stress fractures have the nasty capacity to recur and become larger problems if not treated.