[tremon]

The imperative mood/future tense ("this commit will...") makes the commit the subject of the message, the past tense makes the developer the subject of the message. In my (limited, personal) experience, the people who insist on past tense commit messages have more trouble killing their darlings, are more likely to take review comments personal, and are more likely to view (part of) the codebase as theirs rather than the team's.

So for me, it's not about the commit message itself, but the communication style says something about the developer and how I best approach them.

My take on this requirement is that it helps developers let go of their attachment to the code they've written.