Use 'body' field of page for php code or create a module?

Hi there,

I've been entering php into the 'body' field of a 'page' to implement the site-specific functionality of my site (for example to access and display data from a custom database), while relying on drupal modules/db for general website tasks.

Is this a decent plan? Or should I really be making modules to implement my site-specific functionality?
In future releases I want to allow users to enter information into my custom db and view information others have entered into the custom db.

I thought my plan to use the body field for php was just fine until I started reading about the built in security features drupal provides, and wondered if I should be writing my site functionality as modules to take advantage of those features?

What do you folks do? If you've tried both, what are the pros/cons of each?

Thanks!
Thalassa