On your average small GA airfield during the summer I think hi-viz jackets etc have minimal impact. However, if you've ever worked on the ramp at a large international airport on a dark, cold and rainy night then you'll see why hi-viz's are so important
If you're wearing dark blue or black clothing at night on the ramp in a dark corner of the apron on a wet night then it can be extremely difficult to see you through a steamed up or wet windscreen