Amir and I took a look at this in the context of his current work on T28142 (see D601). Because of the unification with the MultiLabel Segmentation plugin, another check is needed for the interpolation area, as interpolation is currently not supported for segmentations with more than one label. So there is the general check, making sure the selected nodes exist and their geometries match up, as well as the check for the correct number of labels (exactly one).
Every time the second check is updated (e.g. when a new label is created) we have to either be certain the first one is currently true, or check the first one again. To avoid adding a new member variable and manipulating/checking it in various places, we decided to just redo all checks together in one place, as this doesn't take too long and is only triggered by user input.
This is captured in D604, which is based on D601 though