Personally, I prefer pole beans to bush beans, at wider spacing. I use 6" (or 2@12") for some P. vulgaris varieties (Fortex, Goldmarie, Wren's Egg) that have sparse foliage, few branches, or a primarily columnar habit. Small limas, runner beans, the more rampant snaps (like Garafal Oro), and the larger horticultural beans get 12". For the yardlongs, I use 18" between plants, for large limas 24". When I do grow bush beans, I plant 3" apart, in double rows 18" apart.

Note that these are the distances I _start_ with for new varieties, based upon past experience. Then I observe the growth habit, canopy density, and yield, and make recommendations for future changes (if any) in my records for the variety. If I have enough space & seed, I may trial several different spacings within the row. Each variety seems to have its own optimal spacing, so there is really no "one size fits all" rule. The same variety may even require different spacing in other soils & climates, or with different methods of culture.

I agree with Gardenlad, beans (and for that matter, most legumes) will accommodate a wide range of spacings. Further apart, the plants tend to be more robust, have more branches, and bear more heavily. Crowded closer together, each plant will yield less individually, but the total yield will be higher. In my experience, while the yield at the greater spacings may be slightly lower for a given area, the _quality_ will be much higher... so it comes down to yield-vs-quality, and which is more important to you.

So to a large extent, the spacing used is a question of personal preference, experience, and space considerations.

I choose to use the wider spacings for several reasons:

Disease reduction - Crowding reduces plant vigor, and the close contact can aid in disease transmission. I grow an heirloom shelling pole bean, Ma Williams. It is a wonderful, high-yielding green shell, but has some rust susceptibility. The vines are rampant; and while it does well at 6" spacing in good years, the dense leaf cover can be a problem in cool, wet summers. So I use the wider 12" spacing to improve airflow through the canopy. The vines respond by branching & bearing more heavily, and the yield is nearly as high, with far fewer plants (and much less crowding at the top of the trellis). If you are troubled by fungal diseases of beans, you may wish to try wider spacing (and mulch to prevent mud splash).

Seed quality - Since I grow many beans for seed, I am interested in the largest, most viable seed possible. In my experience, I get larger seed & fewer culls at the greater spacings. It also appears to promote faster maturity, but I will need to run several trials to confirm this.

Requires less seed - Many of the legumes I grow are heirlooms obtained from swaps, or through Seed Savers Exchange members. Often this seed is very limited in quantity, and I want to get the most from it. At the greater spacing, I can plant "hills" of 2-3 seeds together to aid in breaking through the soil crust, thin out where necessary, and have each plant grow to its fullest potential.

I personally believe that the standard spacings given on seed packets for beans & some other legumes (such as soybeans) are way too close, especially for pole varieties. True, close spacing creates a dense canopy, reducing weed growth & conserving water. But it also provides cover to herbivores & insects, weakens the plants, and increases the risk of catastrophic loss due to fast-spreading disease.