Graceful handling is a trade off. Sometimes the check is expensive, if everybody normally does it right you don't want to check (particularly in a tight loop), but you don't want to lose this. I work in embedded, in production our CPUs are not powerful enough to risk all those runtime checks, but I want to turn them on when I work on my workstation - in hopes that if there is a problem we catch it.

Contacts are also a guaranteed syntax. Runtime checks provide no way for tools to know if the condition is expected to never happen - this is a bug; vs this is a normal user error we need to check for. A contract means theorem provers can find code paths where a bug could happen - of course we don't have theorem provers for C++, but this is seen as a requirement to get them.