George,
I ride about 8-10,000 miles a year on Texas roads. The vast majority of these are FM roads with no shoulder. Most of my riding starts in the D/FW outskirts and heads into more rural areas. Very rarely do I have any problems with traffic. Once in a while someone passes a little closer than I would like, but the lack of shoulders is not an issue for me.
Many times, shoulders are not so great. If the road is paved with chip seal, the shoulder will generally be much rougher than the main traffic lane since it gets very little traffic to wear it down. Of course, as others have mentioned, all the debris tends to collect in the shoulder. When I do ride shoulders, I generally stay to the left side as there is less garbage there.