One of the criticisms lodged against SharePoint is that you install it one day and the next day you end up with thousands of sites. Each site brings with it potential security risks from improper security settings and massive storage consumption resulting from, among other reasons, content dumping.
Even assuming you don't have either problem, it's always better to have a well managed SharePoint environment than one without appropriate controls on usage. That's where site provisioning comes in.
To be honest, there's no magic formula for managing the site-provisioning process on SharePoint. Different companies will have success with different approaches. But there are some guidelines that enable every organization to both provide flexibility to its end users and ensure an appropriate framework to help manage growth, security and usage.
NOTE: Beyond what’s mentioned in the article, SharePoint Solutions has a good tool for helping to managing the provision process called siteProvisioning Assistant. The product provides a nice mix of pre-built approval processes and end user ease of use.