This "kill em all" approach has been used forever, but as we have seen with antibiotics, killing off all the micro-organisms in a host can have harmful effects, since many of these are beneficial or even essential. The same thing is true of viruses, they are not all "bad". 

> Although viruses are most often studied as pathogens, many are beneficial to their hosts, providing essential functions in some cases and conditionally beneficial functions in others. Beneficial viruses have been discovered in many different hosts, including bacteria, insects, plants, fungi and animals. How these beneficial interactions evolve is still a mystery in many cases but, as discussed in this Review, the mechanisms of these interactions are beginning to be understood in more detail.

Although it cannot be denied that
viruses have caused extensive disease and suffering for
humans and domesticated plants and animals, there are
many viruses that are clearly mutualistic. Some
are essential for the survival of their hosts, others give
their hosts a fighting edge in the competitive world
of nature and some have been associated with their
hosts for so long that the line between host and virus
has become blurred.

SEE:

The good viruses: viral mutualistic symbioses
Marilyn J. Roossinck
Nature Reviews | Microbiology Volume 9 | Feb 2011

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