i think the main reason people dont really want to give flakes is because its hard to control the amount per feeding. its inconsistent, and sometimes its hard to tell if you are overfeeding. also, betta mouths are designed to kinda just,, 'pluck' something from the surface and eat them, such as bugs or mosquito larvae. with flakes, sometimes they dont eat it becuase they dont nibble at food like guppies, they uh... just gulp it. pellets are designed for their mouths and eating habits, and its easier to know how much you are really feeding.
also you have to think about the nutritional values they are getting, and if its enough..
but if your bettas fine with flakes, then i dont see why not, as long as you have a good control of the amount you are feeding.
dont know if this helps, but yeah.. ><