I could see that MT3 collets would enhance rigidity a little and would allow different length tools to be evened up to reduce the number of times it would be necessary to move the head up and down.

TBH apart from drilling its unlikely I would use a vast range of collets, more than likely could support most milling operations with a limited set of endmills and a facemill/flycutter on the MT3 taper

Will think on it some more...