Recently, we've added more-formal uptime monitoring to our review process, and the results show that most web hosts do an excellent job of keeping sites up and running. Even if they get everything else right, sites with uptime problems aren't eligible for high scores. All services suffer ups and downs, sometimes for reasons beyond their control. Those sites that fail to address the problem in a timely manner are penalized accordingly.
The force driving server virtualization is similar to that which led to the development of time-sharing and multiprogramming in the past. Although the resources are still shared, as under the time-sharing model, virtualization provides a higher level of security, dependent on the type of virtualization used, as the individual virtual servers are mostly isolated from each other and may run their own full-fledged operating system which can be independently rebooted as a virtual instance.