sleeping bag only camping

Try googling "cowboy camping" that should give you relevant results.

Yes, I do it all the time. If you are at lower elevations and the forecast is sunny sunny sunny, then you probably don't need a shelter. I usually have a ground sheet with me though, so if I needed a shelter I could rig that up and just deal with wet ground.
Sometimes I will go without a sleeping pad and improvise ground insulation with natural materials.

I did it quite a bit as teenager in the mountains/ back country as kid and once in while on surfing trip on the beach. The general rule of thumb I used for shelterless camping is if the weather report says no rain, I am only walking 2 to 8 miles out I will do it. Any were past the 8 miles out carry a shelter because It will take you more than 2 to 3 hours to get back to your car or home in the dark rainy conditions.
Terry