Actually, historically, Betta imbellis was used for fighting in Thailand in regions where they were more common than Betta splendens. As paradoxical as it may seem, however, the species itself is generally more passive and less aggressive. Betta imbellis is colored similarly to wild Betta splendens, and is often quite hard to distinguish from a Plakat Betta splendens. The species are indeed so similar that they are capable of hybridizing rather easily, and producing fertile hybrid offspring. Historically, again, in Thailand, Betta splendens and Betta imbellis were often hybridized to produce an ideal fighter, though the particular goals involved in the pairings is not known to me.
There are other species of Bettas, however, that do quite well in tanks with their own species. Betta brownorum (extremely shy, extremely rare, and quite the delight), Betta bellica, and several others. In fact, I have a pair of fish of Betta genus, unknown species, that were bought by the LFS as Betta imbellis, but clearly aren't, who are quite peaceful. I keep the pair together, and firmly believe that one is male, and one is female, due to various characteristics and behavior. They are living happily in a tank of splendens females. They hold their own, and do not start fights.