Absolutely keep adding it...it's good for their bottom line. Every manufacturer, retailer and pond professional selling bacteria in a bottle will tell you it's needed or helpful.question though beneficial bacteria- i was following aquascapes instructions to add a scoop when u fill the pond then two weeks later, which i did. should i keep doing this for the rest of the summer?
There was a study, like in the 1970's I think, which kicked this all off that showed adding bacteria did have benefits. However a bunch of studies later, trying to verify that first study, showed there was no benefit. And as Shdwdrgn said, showed it fed harmful bacteria, the bacteria that kills and eats good bacteria. A single company was able to ship a viable bacteria in a bottle but it had to be refrigerated. They said it could jump start bio filtering by a day or two under the right conditions. The right conditions are generally when a new tank is filled with a lot fish and they need that extra day.
Sellers of bacteria will often cite that first study and ignore all the following studies. Same with barley straw and many other pond products.
In science an initial study is considered unproven until other studies can duplicate the results. Remember when Dr. B. Stanley Pons of the University of Utah and Dr. Martin Fleischmann of the University of Southampton announced that they had not only achieved hydrogen fusion in a simple electrolytic cell, but had obtained a substantial yield of energy. Power your home with free energy from a mayo jar. Quickly disproved by follow up studies. I could sell cold fusion mayo jars to the public and cite the first study "proving" it works and that I'm not a con man. Totally honest...right? Same scam is used for many pond products. The amazing thing is thousands of people claim it works...though they never have any data at all. Which is a proven human behavior called the Placebo Effect. They get really mad when their claim is doubted. Better to seem right than be right.
But, if you still believe the sellers there is also a common sense approach that bacteria always quickly grow to the limit of a given system. Since a system is always at maximum, adding more really doesn't do much. There is a short window when this isn't true, like a sudden increase in ammonia, it does take the bacteria a couple of days to catch up.