The only difference compared to joint smoothing in Segment Editor compared to Model maker is that joint smoothing result is converted back into labelmap representation. This makes some very little difference in the final surfaces.
Maybe in the joint smoothing method we could add the option of changing master representation to closed surface model. Then we could store the joint smoothing result as is, instead of recomputing it from the labelmap. Since switch to labelmap representation happens automatically when segment editor tools are used, maybe we could always switch the master representation.