OK, I admit to doing this sometimes. Flattering to my body form thay are not. 'Corse at my age it doesn't really matter.
I use my smartwool pants and shirt exactly as you describe at times. Usually I bring a second long sleeved UL Ibex or Smartwool shirt for hking in. I have been doing that for the past 10 years or so. I can confirm that they do act like a wet suit in heavy rains.
However, in the ADK's at least, they soon develope holes. Getting these layers off while wet, under a tarp is no fun, and putting on a wet sticky layer(after rinsing them out)is difficult. I often will put my thumb or finger through the fabric. And after 5-6 hours of steady rain, they start chafing in odd places, usually at the seams, somewhere. My lower back, underarms, groin, and chest often get very sore after two or three days. I look at these as more helping with about a day of wetness or occasional rain storms over the course of a day than a long term solution to being wet. There is no replacing a rain jacket in two or three days of steady rain when it comes to chafing.
A friend, a semi-professional bike racer, has tights that I tried, but these are actually worse for wear and tear than smartwool. They snag on leaves quite easily and are a mess to clean up after a full day of hiking through the ADK's. A single trip often causes major fraying and surface knots where the threads have come loose.
The bugs get very bad in some of the lower areas, mosquitoes being the worst ... especially this year. A second layer of clothing is required for these areas. As you say, they are no protection.
Anyway, the drying is second to none, drying faster than my nylon pants most of the time. I often rinse my long johns in the local creek. This has other implications, but drying them by wringing them tightly and putting them on is a viable methode. As you say, body heat will soon dry them, provided you can avoid hiking in them through wet shrubbery or getting rained on again. If you are getting rained on, it doesn't matter anyway.
Multiple layers, such as at your waist or where your socks cover the bottom, or where your pack is against your back, do not dry well. Often, even two layers, such as shorts over tights, keeps both pieces wet for many hours longer than a single layer. The outer layer stops or limits air flow to the inner layer. Air flow is extreamly important to drying times. But, generally in 40-50F temps, I have found a single layer of long johns to be warmer than the nylon pants, simply because of the faster drying...once you are somewhere dry. A timely piece you wrote, and I thank you.
Regardless of hiking cloths, I always keep my sleeping cloths/camp cloths dry. Longjohns, socks and a down jacket are packed with my sleeping bag. As long as I am hiking, I can tolerate a lot of temp change. When I stop, I WILL get cold at 35F. My tarp, dry cloths, stove & hot drink become my priorities. A fire if possible, but if it has been raining for a couple days, I ignore it. I strip my wet shirt, and slip on my dry shirt. I strip my shoes, socks and pants and put on dry cloths, then I put my jacket on.
Rather than risk my sleeping cloths getting wet, I will often strip and slip my nylon pants back on if I have to leave my shelter for any reason. (I try to make sure I have relieved myself before "hiding" under the tarp.) I used to worry that I looked like a fool under my tarp: in longjohns, cuddled up around a stove, or drinking a cup of cocoa, maybe with my bag around my shoulders. Well, I guess I do, but I don't believe it is foolish to do so if I am also warm, dry and fairly comfortable. You can laugh at me, it IS allowed.