Use source control because neither you nor your team are perfect. The primary function of source control is to ensure that you have a complete historical record of your development process. Having this record, you have the ability to confidently branch out with "experimental" versions, knowing that if the experiment fails, you can back up to an earlier version.
In addition, a good source control system like svn will permit multiple developers to work on the same file and provide powerful tools for reconciling the differences that each introduces.
