What if your use of ABC passes a parameter to it that tickles an incompatible behavior in a particular version of DEF?

Then we hope that ABC is a good module and has a testsuite that tests that DEF works well with all parameters, also we hope that ABC validates input.

If this parameter is an complex object of class PQR, that is possible, that DEF requires/uses that class. So all PQR versions tested with DEF. Or maybe XYZ tested with all versions of PQR.

It's unlikely that both use PQR but none "use"'s it.

On the other side it's possible that a module does not have testsuite (except one "use_ok") at all and works like "garbage in - garbage out" or sometimes just "garbage out". Obviously tests won't help there.

Anyway, better test something than nothing.