Why the hell would I want to tell an organization that is more focused on their actual business that they need to spend hundreds of thousands of dollars to build up a datacenter over weeks / months worth of time when I can literally do it myself using Chef / Puppet and Amazon EC2 in a few days, and we're not on the hook for any hardware maintenance or replacement in the future?
The business gets to keep focus on the business without the overhead of running a whole datacenter including power, cooling, wiring, real estate, countless admins, service contracts for hardware and network gear, construction costs, built-in costs for future hardware replacement and scaling, etc. etc.
There's a reason why lots of people are following Amazon into this space. It's possible to do things right, and to do it cheaper. And you are far more agile in needs should you be successful by pairing their load balancing services with something like Chef or Puppet. Oh, and just do your offsite backup out of "the cloud" to a box at your office, and an off-site at a regional or whatever.
Yes, there's some risk associated with the "Amazon / Microsoft / RackSpace / Whoever fucked up", but it's far more likely they'll figure it out and get it back up and running far faster than if the same fuckup occurs within your private datacenter, because datacenter is their business while the company I'm working for cannot say the same.