While you can move, say a MIDI bass channel from an imported file to the Bass track, you have to be careful, and it's not really the way BIAB was designed to work. If you copy it, you will need to freeze the track; otherwise, it will get overwritten by a style re-generation. And then once frozen, it will not adjust to any changes you might make on the chord grid (as it would from a style generated track).
As stated, a DAW is probably a better place to do those things.
John, you're correct on every point except . . . you're omitting the situation where it's the perfect tool for the job, limited as it may be. The only actual error is "If you copy it, you will need to freeze the track". I've used this tool often over the last few years and BIAB automatically freezes the tracks on my program. This tells me it's intended to be used after all editing and chord grid chart work has been done.
Otherwise, all you say about the tool is correct. But it's only correct at the top level. It's my opinion this tool does exactly what PG staff programmers designed it to do and case in point, it does exactly what the OP questioned if BIAB can do. It doesn't address the issue of generation and regeneration. I think BIAB programmers designed and included the tool for situations when the chord grid will no longer be edited and modified. That can either be an original song, manually input cover or even perhaps an imported commercial, high quality midi file that will be used without modification and the project is at the point of rendering and the user wants as high quality and realistic sounds in the render as possible. Move/Copy works superbly well in that limited circumstance.
I've always thought the tool was designed to provide the user access to the higher quality HI-Q patches and better synths, patches and third party plug ins to enhance the quality and sounds for the instruments. Anyway, that's what I normally use it for.