Margo has been quoted in and out of context as saying that "sometimes the best option is to just get wet". When applied to biking, my take on this dictum is that the "sometimes" is when you know how and when you will get dry and warm. For example, if you are only a few minutes from home, it probably isn't worth the effort of stopping to don your rain gear. On the other hand, if you are just setting out on a long of biking, it is probably wisest to deck yourself out.
The type of rain encountered also matters: a light rain in warm weather probably doesn't necessitate full rain gear whereas a downpour in November calls for everything you can lay your hands on! The presence or absence of wind also matters greatly as being wet will amplify the cooling effects of the wind.
This is the background for some of this summer's commutes to work. The weather has been quite hot and humid making rain gear unpleasant to wear on my daily commute. Consequently, I have been biking in the rain with minimal rain gear on several occasions. The most extreme was one day when I stuck my head out the window to see that there were some sprinkles, but no major rain in sight. Consequently, I pulled on a quick dry t-shirt and stuffed a cotton on in my bag and set off to work in shorts, knowing that I had long pants at work. After about 10 minutes, the rain began to change from sprinkles to a serious downpour by slow steps. It kept looking like it would ease up, only for the rain to increase in strength! By the time it was a total downpour, I was thoroughly wet, but as I was only 5 minutes from work, I didn't stop to pull on my rain gear!
I got into work looking like a drowned rat. However, I could change out of my wet clothes into dry ones once I got to work. I had mentioned that I had a cotton t-shirt in my bag. Well, I also had respectable shoes, long pants and long-sleeved shirt in my filling cabinet. The shoes are there all the time, and I often leave long clothes overnight during the summer. In addition, for over a decade I have kept a pair of socks in my filling cabinet for just an event. Thus, I was quickly able to get into dry clothes.
At the other extreme was yesterday. When I left work, there was about the lightest rain I have every experienced: tiny, fine drops. It was an utterly warm rain. I gave no thought to rain gear and pedaled home through the damp air.