I think people simply like attractive looking fish, and sometimes giving your lonely little betta a friend doesn't seem like such a bad idea. That said, i have never added anything to my tank with my male, i definitely think he's a lone ranger. BUT i did decide to try some guppies out in my community 25 gallon tank with two female siamese (not because i thought i would 'take the risk' but because i honestly didn't think it would be a problem). I added 5, within 3 hours, i had lost 2 and the very next day, as i was trying to find a large separating net to separate my new guppies (as i was terrified for the lives of the other 3) the other 3, all female guppies were also killed at the hands of my females. I will never make the same mistake again. I have heard however, with the right amount of space, female siamese are generally pretty ok with anything, as long as you mix up the tank before adding anything in with them (they are very territorial, especially to each other).
Well thats my bit
Prue