I think eventually a lot of M&B mods will be ported to Warband. However, it could take many months at least for some larger mods to be ported, especially as Taleworlds are still patching Warband, and so many modders are hesitant to start porting/modding in earnest until they are confident that they won't have to port to a newer patch of Warband. If you can find M&B cheap, then you might as well get it for the mods.