They do and have raced in the rain, but conditions could warrant a delay of the start or cause a red flag to be issued with a later restart of the race, assuming things got really bad. Visibility tends to be a big issue as all but the first car or two are enveloped in clouds of spray generated by the cars in front. Prior to 1971, the F1 cars ran treaded tires in all conditions. Once slicks were introduced in '71, treaded tires were relegated to rainy conditions and it was not long until rain tire technology began to develop. You can see (barely due to low resolution) Depailler is running treaded rain tires in the Montreal clip.
This official F1 video covers a few more notable rainy races over the years.
Correct. Truth is, given the light weight of the car and the power produced by that lovely (and distinct) sounding Ford/Cosworth DFV engine, they could spin the wheels in almost any gear in the dry if not for precise and judicious throttle application, but the rain makes it almost impossible not to do so.
I suspect he was both testing track conditions and having a bit of fun as well. Some drivers hate the rain, others love it and excel at driving in the wet. Generally speaking driving in the rain was/is easier on equipment, provided one didn't crash, which was a pretty big deal when F1 cars were much more fragile and less reliable. To finish first, first you must finish!
Always a possibility, he was hydroplaning quite a bit on the straights as well.
Very. Excellent choice of words.
For whatever reason, most American race series do not run in the wet, though many of them don't lend themselves to it, either. GT and Le Mans related racing series will run in the rain as does MotoGP and BSB. As Grand Prix, USRRC, Can-Am and Indy legend Jim Hall once said when talking about living in the UK and racing during his brief stint in Formula One, "If Englishmen don't race in the rain, they don't get to race." True words from a man who knows how to drive quickly.
I always liked Depailler's style, he was a bit of a French hoon.