Very nicely said, Wuv and SRC. John, I know that you have kept bettas a certain way for a long time, and it has worked, but I think that writing off the advantages of a reasonably large tank as being strictly emotion-based is a little rude; there were some definate valid points about temp stability, behavioral enrichment, etc. Yes, it is a personal choice, but BOTH housing options have thier benefits. Obviously, no one is suggesting you keep a highly territorial fish in a MASSIVE tank, but most people would probably not consider 10 gallons too outrageous for a fish that is a good 3 inches long.
I don't really think that keeping bettas in larger tanks is a strictly emotional choice. I think that it is generally accepted that the bare minimum is just that; the absolute lowest level of care for health and survival - as in not the ideal, but the acceptable. You can keep a dog completely outdoors on a tether with fresh water and food, bring it to the vet for appointments when it is sick, and keep it on dewormers. It will live a good long time. But, seeing as dogs are pack animals who need attention and interaction, I think most people would say that the dog has only been given the minimum it needs to survive. Obviously, bettas can survive in much less water than other fish. But it doesn't necesarily mean that they are stimulated, well-exersized, or in ideal conditions. Now, none of us can go out and buy a rice field, patch of flooded grassland, or wetland for our fish to live in. However, I think we can safely assume that for at least a decent portion of the year (from what pics I've seen, anyways), the fish are in water that is at least ankle deep, but certainly containing a greater volume of water due to the length and width of the area than your average bowl or small tank. So, I would say that "ideal" would not be so much a particular aquarium, since most are a bit deep for a betta's tastes, but a fair-sized tank filled to maybe 5-10 inches deep with a good long length and width. I plan on keeping my next betta in a 10 gallon tupperware, since it is very long and wide, but pretty shallow as well, for example. Add to that live plants, blackwater extract, etc. and I think I'll finally be getting at least closer to what the species needs for survival, as well as its less essential but still important requirements.
However, I do agree that each betta is different. Some are very territorial and insecure, and need confined spaces to feel comfortable. Others hate being in small containers (take my betta Matamoras for example; if you put him in a cup or smaller tank, he freaks out, slamming against the walls and trying to jump out) and need plenty of space. I think that in addition to personal preference, finnage, age, health, etc. should all be considered; obviously, you do not want to move an old, super huge finned betta who just got over a nasty infection into a 10 gallon tank, as he will surely suffer from the sudden stress of his surroundings.
That said, I have NO issue with breeders keeping thier bettas in small enclosures. Frankly, it would not be financially feasable for them to be kept in anything larger, as others have pointed out, because of the massive size of spawns and the huge volume of fish kept. That, and if you are a good breeder with desireable stock, they shouldn't be around for long, and will hopefully be off to homes that will keep them in at least a gallon of water. ((I've found one gallons too difficult to keep at a stable temp, as well as just to cramped looking, but I think 2.5's work nicely for isecure bettas, and 5-10 gallons work well for strong, bold fish. But, apparently they do work for some people)) So, I doubt anyone here is casting stones at the breeding community.