So there are really two separate issues here:
- Is it useful for hosts to be able to change maps in the lobby for their games?
- Are there reasons this should not currently be implemented? / Are there other changes / improvements that need to be made to properly support this?
I think, undoubtedly, the answer to #1 is "yes, there are obvious situations where this is useful".
The problem, at this point in time, is #2.
There are complications caused by how downloading mods / maps, and handling changes, are currently structured in the game. There are complications in how the lobby deals with this. While in the "normal case" these things may (sometimes) work, it opens the potential for serious issues that need to be fixed.
For these reasons, I'm currently a "no".
But I am also working on improvements for these things.
Once those underlying issues can be improved (as well as the user experience surrounding how changes are handled in a lobby - we presumably need to reset "Ready" state and make sure everyone is aware of changes and can opt-in/out to them), I could see switching to a "yes".
So this isn't as simple as "can you think of reasons this feature is useful", or "is it useful". There are things that need to be fixed to properly support this feature at this juncture (some of which are already in bad need of rewrites).