From experience most web designers are only interested in making the site pretty with little thought of usability or any understanding of code.
This they will stick any old code in without checking it worked. Many designers I knew used to disable the script error pop up so they wouldn't see the error...
The best one was a site that complained they never got any messages from their contact us page.
I pointed out that the submit button had no script associated with it so iit would never send an email