Re rain gear, I'm completely in the corner that believes that full rain gear makes all the difference, rain booties that rain pants go over mean your shoes aren't soaked, and a helmet cover means rain isn't pouring down your face and neck.
Sure if it's super hot out it's not a huge deal being wet, but I've ridden in rain and storms tons of times where it's 20c or less , 68f or less, and you get miserably cold fast--and I'm talking summer months, July August.
Different strokes... I always figured 50F and raining was fairly pleasant. It might get a little chilly on the long descents, but I can still work up a sweat on the climbs or even the flats.
I always figured that wind chill protection was the main thing more so than actually keeping dry. I take clothing that is warm when wet and a windproof shell that goes over it. I come from a whitewater kayaking background so being wet in anything above freezing was normal for me in that sport and I carried over some of the clothing strategy where I just expect to be wet.
I sometimes have ridden in sub freezing temps on tour in summer in the mountains but almost never much below 50F with rain. I am not sure at what point I would take a day off for a cold rain, but it hasn't happened yet.