I guess it depends on the level of ambient light. The flash duration is very short so there won't be any motion blur if the ambient light is low and most of the illumination comes from the flash. If the level of ambient light is high and you use the flash only as fill-in, then, in theory, you could experience motion blur - but that's not a likely scenario since you will need high sync speed (and therefore CS or HC lenses) in the first place, unless you want to shoot at f16-22 all the time (or use ND filters), otherwise the high level of ambient light will result in overexposure. The ND filter can be a good solution if you don't have CS lenses (you can watch Joey L's film on his portrait project in Ethiopia where he uses a Phase One body/digiback with a battery operated studio flash and balances the ambient and flash illumination using an ND filter). In summary, if you shoot in a studio and all illumination comes from flash heads, you don't need to worry about sync speeds and motion blur. If you shoot outside, or elsewhere with high level of ambient light, you need to use either high speed sync (CS/HC) or ND filters (and you may indeed experience some motion blur with the ND filters).