As everyone probably knows by now, I am not keen on keeping a betta in anything less than 2.5 gallons, and strongly prefer they be kept in 5-10 gallons. Most bettas I've seen are about 2-3 inches, and I think that even though the inch-per-gallon rule doesn't apply to them due to thier low waste production and ability to air-breath, they still deserve at least a bare minimum of space. I seems very difficult to heat, filter, and decorate bowls and smaller tanks without taking up most of the animal's swimming space.
Many bowls are less than a gallon (which is considered the absolute minimum permenant tank size for bettas), have no lid, and are too small for heaters. They provide very little living space, and more importantly, are very exposing with little opprotunity for behavioral enrichment.
I am a person who believes that fish are somewhat intelligent animals, who get just as distressed as any other creature by being cramped, overexposed, and unable to entertain themselves. Whenever I see a betta who lives in a cup or bowl, he or she just tends to... sit there. They always seem very bleak and inactive, with poor muscle tone and sluggish movements. In any other animal, we would suggest that the animal is bored or unhappy, but most people refuse to anthropomorphise fish by suggesting they are intelligent enough to be poorly stimulated.
On the other hand, I keep all of my bettas in 5 or 10 gallon tanks, and everyone is always shocked to see how active and inquisitive they are. I've heard the argument that bettas are afriad of big spaces, and will just sit in one little corner of a large tank and be miserable. The only time I have ever observed this is when a betta is in a large community tank, and feels compelled to claim and defend one small territory within the tank. Otherwise, bettas kept singly in 5's and 10's are always zipping about, sneaking through thier plants, or sitting in thier caves. Each of my 5 bettas uses every inch of his or her tank; they never stop moving, and seem very content. I periodically change around thier tank ornaments and plants, then watch them explore every inch of thier environment; it will keep them entertained literally all day.
My only exception to be "2.5+ gallon rule" is breeders, primarily because most of thier bettas are temporary residents, who will not live in a jar or beanie baby box for thier entire life. I understand that in order to breed healthy, beautiful animals for us all do buy and love, they need to be frugal in purchasing housing, as well as space-efficient thanks to the massive broods some bettas create. I have a great respect for reputable betta breeders, because they offer us a source of healthy, beautiful fish, so we do not need to support pet stores who generally have absolutely abhorrent betta care practices.
In short, I am of the opinion that if you are financially able to provide a better life for your animal, you should. And, if you can not afford the bare minimum of space for that animal, you shouldn't buy one. Fish are just as much a living thing as a bird, hamster, or dog; they deserve to have thier basic needs met, and exceeded when possible.
Sorry!! <<
. I'm planning a overhaul on my room, and probably will stop keeping community tanks, and instead move on to small species tanks. For instance, I'm planning on dividing my 20 into two at some point and having a betta in each with maybe some loaches (I'm thinking up an ingenious divider that lets the loaches through but not the bettas 
