Believe it or not, I've had yardlongs volunteer for me fairly often... which is surprising, given my cold winters and wet springs. Other Vignas (adzuki & mung) have occasionally volunteered as well. In contrast, Phaseolus beans never volunteer for me; I assume they just rot in the ground.

Last year I had several volunteers for "Yancheng Bush". Unfortunately, the ones I tried to transplant didn't survive. But one of them was near the area where I had intended to plant them again, so I let it grow. It was by far the healthiest plant in the row, and produced a large amount of seed.

I'm not sure if seed saved from the volunteers would be significantly different from the original seed. To select for a trait, it has to be there in the first place... and since beans are self-pollinating, they tend to have a narrow genetic base. This is especially true if, at some point along the way, seed was saved from a single plant.

However, it certainly wouldn't hurt to try. At the very least, the plants may be healthier, and produce larger, more viable seed. If you grew more than one long bean & the different varieties crossed, who knows? You might get something new.

I've even had some Phaseolus beans volunteer. But, I don't know if the seeds survived in the ground over the winter, or merely fell to the ground during the early spring cleanup as I try to strip the old vines off the fencing and bamboo poles.

But, yes, long beans seem to volunteer.