I'd guess most people assume open source is cheaper, hence don't even think about a bespoke system. You ought to give an out of the box Joomla or Drupal a bash, they're really not that difficult to get on with, you may even like it, however, you will then be able to talk with authority when you compare to your CMS
Too right. This might be part of a very early stage of research, but don't just take what "some guy told you on the internet" as gospel when making an important decision! For my 2p, we also have our own in-house CMS (which we are hoping to open up for public use very soon), however, if I was pressed to pick one of the "free ones", so to speak, I'd plump for Wordpress as code-wise it seems the neatest of those I've used.
I like Content Management System (CMS) as it would allow you to change your website's content with easy to use interface and i agree with Alex as it would not require any programmer to do so.