Oh no. I feed a variety... freeze-dried, frozen worms, shrimp, etc..and I am even considering a bit of frozen raw hamburger on occassion. I know a breeder that feeds frozen raw hamburger and his fish look absolutely fantastic! They even look "beefy", LOL... I'm in the process of discussing it with him before I try it.
The pellets are the basic daily food, alternating other "richer" foods. Like with any animal or fish, you need to introduce the other stuff incrementally because of sensitive digestion. If they aren't use to it, you can really cause problems feeding new foods.
Like the girl who had an ant problem and the ants filed right into her fish tank and her Betta hog out on them. He became severly bloated and died a two or three days later.
In the wild, they are use to eating all sorts of things that fly, swim and crawl. But our guys (and gals) are domesticated, so we are trying to copy the same sort of lifestyle.
I'm extremely careful when feeding new fish... I have no idea what they've been fed or if they've been fed on a regular schedule. I ease them into the basic diet and increase amount slowly over weeks. They act like their starving and want more than a couple pellets, but I'd rather have them a little hungry at first and get them regulated on my food before increasing it or introducing worms and shrimp, etc...
Of course, Bettas always act like they're starving... "More, more!" they seem to say. They can be real pigs! So ya got to be careful in the beginning.