The difference to me is that the cutter load is random in position and loading as well as intermittent, the belt is constant in position and load, it may well work out but don't forget these spindles are seen as disposable - they are pretty much dirt cheap and a set of decent quality matched bearings will be around 2 x the cost of the spindle whole - there is no free lunch.

Preload is a subject in itself, it should be documented somewhere on the bearing manuf. sites.