Would you expect that processing filters will work well, too?
For example, for the provided sample data set, thresholding works for the first scalar:
But you get garbled results if you choose the second scalar:
I’m sure that VTK could be prepared to handle additional scalars in vtkImageData (or support can be added at application level), but I’m not sure how much it is supported in core VTK filters, mappers, and IO classes.