The cycle paths around you are great - they go to places that people want to go and make cycling pretty easy..... This is the one I use daily. Still some cyclists use the road. In my case I can't understand why apart from passing other cyclists but from the sounds of it not all paths are like mine...
Why would I not use the path in the picture? well I would in the uphill direction but downhill from the army camp it is down to speed and convenience - The nasty corners associated with the industrial estate service road crossing are a real PITA on a 'bent.
Having moved away I really miss the (now) great cycle infrastructure in Weymouth - it used to be poor to shocking like the rest of the country. The provision isn't universally great now, but it is much better than average.