Limitations and Evolution

That's right! To elaborate: DBC is not a run-time tool, although some toolsets that support DBC provide useful runtime-checking facilities (checking assertions, preconditions, etc. at run time). The main point of DBC is to document precise specifications for interfaces, which allow clients (programmers using the specifications) to use the interfaces correctly and to make it easier to discover defects in software that uses these interfaces (whether by testing or inspection).

Run-time checking of assertions certainly is useful, but often must be turned off in production systems for efficiency.