A bubbler is fine as long as its low enough not to bother them. It breaks up the surface in a divided tank keeping the unfiltered side from developing any kind of film. Also bettas do breathe from their gills, even if its not as much as from the surface, as well as their labyrinth organ so it can be useful as long as it's not bothering them. It really depends on your fish c:
Warm and sometimes still/slow water holds less oxygen so the species developed the labyrinth organ as a way to survive and then it became their main way of breathing.