The continue curve points can be used in two different ways. One is to specify the position of the curve at a system break. The other is to allow multiple bezier curves to be combined together to form complex curves with many inflection points. I don't know any software that is using the latter case, though Dorico sounds like it could do that.
Perhaps it would make sense to differentiate those two cases in the semantics somehow, but that is probably something outside the scope of MusicXML 3.1.
The bezier-x and bezier-offset are there in case an application can specify in musical coordinates but not graphical, or vice versa. I think most everyone uses bezier-x though; bezier-offset may not have been the best idea.
Any suggestions for documentation improvement are most welcome at the W3C Music Notation Community Group!
VP of MusicXML Technologies