[support] HTML forms not Drupal's Forms API -- Badness Explanation Needed

Gang, I've red-flagged for a potential client (abandoned by former Drupal developer) an event registration page on their site that uses an html form to deliver some variables to a Paypal page. Looks like the code came from a Paypal help page. I'm proposing using a combination of the signup and signup_pay modules to handle this functionality. I'd like to give the client a little more detail on why it is bad to use regular html forms in addition to "It's more stable," or "it's the Drupal way." On the "It's more stable" argument, I need some more detail on why and what are some bad things that can. Any help would be most appreciated. Thanks, Shai