Comment 2 for bug 1087344

For honouring a commit message that the user has set, the rationale is straightforward-- the user wanted this commit message to be used, so the tool should use it.

For setting the commit message, there are two reasons:
Many projects have different conventions for commit messages and merge proposal descriptions. The commit messages tend to be short summaries, while the merge proposal descriptions are long, nuanced descriptions that are addressed to a reviewer and are intended to understood in the context of the diff. Examples include Bazaar and Launchpad.

Pragmatically, tools such as Tarmac expect this value to be set, and so not setting it makes it harder for users of those tools to land their changes.