The advantage of "use Inline C => ..." is that it happens at compile-time when there is still a chance to declare functions for use further down in the module. Otherwise, perl code later in the module (as it is being compiled) won't know that the XS functions exist, forcing you to use parentheses when calling them.

This advantage might not apply to that many people, but I'm guessing that was why it was done as a BEGIN behavior.