My main page has a intro image beside the main content.
I would like to add that image using custom fields.
I've noticed that pages don't have. I would like to enable them.
Is it a bad practice?
Pages do have Custom Fields enabled by default on WordPress. If you're not seeing them on the "New/Edit Page" page, then check the "Screen Options" pull down tab on the upper right corner of your Dashboard. See if "custom fields" is checked.