I don't have a Milling Machine, so I use the MT3 taper in my Boxford headstock pretty much all the time. I've just spent a happy 2 hours in the barn swapping out an 'Optiset' centring microscope, a centre drill, a #50 drill and my crappy tapping tool 128 times, making the cylinder cover holes in four G scale loco cylinders.

Optiset in a 1/2" MT3 collet. (Does anyone else have one of these? Got it off ebay ages ago)

Crappy tapping tool – just an MT2 blank arbor and a drill chuck from 'Princess Auto'

Another eBay bargain – nice little boring head, being used as a flycutter.
If I ever get a mill, it will have to be an MT3 one, as all my tooling is this size.