As others have stated there is a host of prior research on this.
I have done some modelling and simulation research, which shows that the presence of non-host bacteria (i.e., not the target of the phage) can scupper its attempts to fight the pathogen. I called this effect the "decoy effect". In complex bacterial ecosystems such as the intestines, the harmless bacteria can easily outnumber the harmful ones by a huge factor.
There are quite a few observations support this idea, showing that phage treatment, or treatment with bacterium-eating bacteria such as the wonderfully named Bdellovibrio bacteriovorax (there is also a Vampirococcus) help most in those cases when the host microflora is absent or is outnumbered (see M.H.F. Wilkinson, Predation in the presence of decoys: an inhibitory factor on pathogen control by bacteriophages or bdellovibrios in dense and diverse ecosystems. J. Theor. Biol., (2001) 208:27-36. Pre-print version available in PDF (292 kB)).
This does not mean the opportunities offered by phages should not be researched. We should not expect them to solve every (bacterial) ailment. Personally I think we will keep having to find new antimicrobial strategies. It is a case of what in evolution is called the "Red Queen Effect": you have to run just to keep in the same place, in terms of fitness.