I suppose technically you could just leave the blade spinning while you load logs or take flitches off. I tend to throttle down after a cut just to keep the revolutions of wheels/blades/bearing to a minimum. Some guys never throttle down, even when clutches are used, but only at the end of the day or for lunch or something. Personal preference I would reckon.
I can see on a manual mill that safety could be a concern. A blade spinning around is dangerous as hell when one is turning a log or something in front of it. One mis-step or losing balance could be a horrible situation if you fall in the wrong direction. On a powered mill all the functions are so quick and easy, leaving the blade spinning might be easier on everything. I've seen it done both ways and have done it both ways.
I can see on cold days where trying to pull start an engine with all the wheels and bearings and such trying to spin in the freezing cold would be impossible so a clutch is mandatory here.
In the end, all I know is most every mill out there has a clutch of some sort, whether centrifugal or belt tensioned or electric PTO, or....? For whatever reason apparently it's just a good idea.