NoD, my present job involves flying the private owner of the aircraft; I'll say no more than that.
Yes, bad weather / IMC does reduce the number of aircraft in the sky, sometimes what a welcome relief that brings!
This will possibly sound selfish, but it's not meant to be. From my point of view, the very worst time to fly is in the daytime, in excellent weather, especially at weekends or on bank holidays.
Hordes of low time non-IMC rated pilots who have been waiting for weeks for good weather are trying to get current; they are often thinking more about the physical control of their aircraft, navigation and what they are about to say on the radio than on lookout. Solo students are trundling along on their QXC flights, ditto. The radio workload of the ATC controllers can be exponential at these times, sometimes a meaningful ATC service is unavailable. This forces more aircraft to route round CAS (can't get a timely clearance) making the choke points a very dangerous place to be (Luton's western and eastern ends, for example). A re-route puts many pilots' eyes inside the cockpit, or down to the surface, instead of remaining up front....looking out.
I have had a number of airproxes in my thirty years or so of flying (airmisses as they used to be called), all have occurred in very good weather and in Class G. Aircraft colour contrast is often not good in sunny weather, despite what many might think. For example, a white or partially white aircraft can disapear against the background of mixed cloud. ANY colour of aircaft can be indistinguishable from the mixed backdrop of a town.