This may not be installed on various systems, and it's difficult to test
for the availability of the tool you need, if the check program itself
does not exist.
The POSIX 2008 specification mandates the `command -v` builtin; bash is
a POSIX 2008 compliant shell, and this builtin has worked since bash 1.x
anyway.
A side benefit of using the POSIX portable option is that it requires
neither an external disk executable, nor (because unlike "which", the
exit code is reliable) a subshell fork. This therefore represents a mild
speedup.