I'd like to point out that, while you aren't breeding, bettas are at all times aggressive. With each other, with other fish, with frogs, with snails, they simply are. Females and males alike. Just because you had luck in the past doesn't mean it will keep on going. Youtube it. I'm SURE you'll find PLENTY of videos of bettas attacking each other, and other animals in their tanks. Heck, there's a video in the picture forum of a betta that's lived in harmony with some frogs for forever, and then when she was filming he freaked out and attacked the frog. It was a hard bite, meant to hurt. If that was another betta, male or female, they would be in some trouble.
Another point is females are supposed to be less violent, thus no fighting. Untrue. Many females have to be removed from sororities due their overly- aggressive nature. Females can be just as, if not more, aggressive than some males.
In conclusion, I was actually thinking about this today, the whole, "Well, it worked for me in the past!" excuse... The question really boils down to "IS IT WORTH IT?" Is it worth risking the life of your fish? Or are they simply that... fish, to you? For 7 bucks more you can have a 2.5 gallon critter keeper, even less if you buy it used. You can even find plastic shoe boxes or plastic containers around your house to put them in. It's look to me like this is more of a case of being stubborn than thinking about your fish. Are you thinking about them? Or yourself?