Using BibTeX when submitting an article
I'd send the .tex and .bib file, which seems by far the most logical thing to do, to both you and me at least. On explicit request I'll grudgingly provide the .bbl file.

This should make things easier for them, as you correctly suggest. It is easy and fast to convert a .bib to a .bbl, but the other way round is impossible.

If the journal has a workflow that can't handle .bib files, that seems their problem and not mine. They are the professional publishers who charge for the "added value" of the professional typesetting and printing service, right?