Well the imagesystem isn't only for booting dos so people might be able to use a more stable win3.0. As far as I'm concerned it's for booter games. That it allows people to run another version of dos inside dosbox is side-effect.
About version numbering. Dosbox doesn't run all games yet. (some people think this might even be impossible) So there is no reason to anounce a version 1.0. Some applications claim to at version 3 or 4. So their goals must have been reached multiple times. (although I have reason to believe that not everybody follows this way of numbering)
Each version should aim for improvements in that area. If dosbox runs a stable number of games(it doesn't increase anymore with new versions). I think It would be nice to call that version 1.0 and let all enhanchements which do not add to compatibilty be added to that version. Although I understand and enjoy the need for stuff like graphical filters and such.
To run win3.x real good, support for filesharing must be added. At the moment I haven't encountered a single game which depends on that (Ms office and such do that), So it has little to no priority to add that to dosbox.