> bacteria and human cells are drastically different

Doesn't that sort of depend on what you consider a human cell? Our gut bacteria is supposed to be fairly important to us, and something that might affect it on a large scale might have consequences.

Note, that I'm not saying that bacteriophage use if harmful, just that you have to be careful when discounting possible harmful side-effects. History is rife with examples of new safe health technologies that we only understand the negative aspects of later.