Not all bacteria are harmful. This phage therapy needs to not wipe out the symbiotes that make it possible for us to stay alive.
Actually it's going the other way around :
- Antibiotics are rather indiscriminate and can kill large swaths of bacterial population, including commensal flora (= "the bacteria which normally live here", i.e.: non-dangerous).
That's one of the reasons (the non-ecological/resistance one) why doctors try to avoid over-prescribing. (Just ask any girl who got yeast infection - e.g.: candida - because her flora got disturbed by a wide-spectum antibiotics)
That's also a reason why antibiotics can be prescribed with micro-flora supplements (the antibiotics will kill the commensal flora in addition to the bacteria causing the disease you're trying to cure, so you need to import new microorganisms to compensate - usually Saccharomyces, a type of benign yeast)
- Phage are the bacteria equivalent of viruses. They target *specific* surface receptors. It's like viruses and eukaryote (you might catch flu from a swine because surface cell receptors are close enough for a virus targeting one to be able to bind the other - ie. we're closely enough related. You'll probably never catch a virus usually affecting plants.)
A phage might be able to recognize and bind a few related bacteria, but will never affect other completely different prokaryotes).