Well, some virus-es do stay in your body for life, like chicken pox. Old people who had chicken pox when they were young often get shingles and post-neuralgia syndrome, which is a resurgence of the virus, like the Taliban. My dad had that, and you don't want it. You can however develop immunity to the symptoms caused by a virus and most people do.
I don't think virus exists to be useful or not useful. I think its just a natural result of DNA mutations. Sooner or later any bio-sphere is gonna develop virus-es. When you look at what genes do for a living, all that chopping and splicing, it makes sense that all manner of weird stuff will happen.