Here's some pontificating:

Viruses form a part of a control system for themselves and for higher and larger life forms. If any system becomes unstable, or if one organism starts dominating or growing out of control, the other organisms have an evolutionarily created self interest in bringing things back into balance. Viruses play a huge part in this self regulation mechanism.

Eliminating viruses would have the unfortunate effect of decreasing overall system stability for biological systems at every level.