I want users to be able to submit nodes and comments via AJAX. I also want to do some fairly extensive customization of the node and comment forms.
I've spent time looking through documentation and code examples for Drupal 7's Form API and Ajax framework, but I find it very complex. Therefore, I simply want to create my own form in HTML and use my own JavaScript code to submit it via Ajax. I'll also set up a specific URL for processing these Ajax requests, which will ultimately call node_save() or comment_save() when appropriate.
What are the downsides to doing it this way as opposed to going through the Form API and Ajax framework? I'm not creating modules for contribution to the community. Everything is just for my own site.
The usual argument would be about portability I guess, but if you're not going to be porting these modules to another Drupal site then I guess that falls down.
The same can be said for offering other modules the chance to alter your form based on some global/inherited setting, but again if you really don't want/need this functionality then it can't really be used as an argument against.
The one thing you will lose out on is the built in Cross-Site Request Forgery protection. As long as you're rolling your own version of that, though, you should be ok.
If you plan to use Drupal a lot I'd recommend getting used to the FAPI though...after a while it actually becomes a lot easier to use the FAPI than write out custom HTML.