I believe the question will shift from "why use cloud computing" to "please justify the extra cost of not using cloud computing." Hardware (or "machines") is only one part of that cost. The other much bigger part are the personnel costs associated with keeping the hardware running and (re-)building functionality that's available in the cloud (e.g. storage, queue, db, etc.).

Plus there's alot of FUD going around in the area of security, monitoring, logging, etc. Amazon's EC2 instances are nothing more than linux boxes, so if your sysadmins can do things like "apt-get install syslog" then all of these things can be addressed.