1) We can ensure that only the fields that are defined by our PHP code can be used.
Currently, you can add html to any Drupal generated HTML form and the values will be processed and possibly be inserted into the database. We can also check if the fields still are of the type that they should. I am not sure that you can gain anything by exchaning a textarea input to a radioselect, but the possibility annoys me.
With the exception of the node, comment (?) and user forms, no forms behave like that. What other forms behave like this?
2) Themes could change the order and placement of fields. You could decide to generate your taxonomy tree somewhere else than between title and body. Maybe you can already do this through CSS, don't know.
If that is the case, we need to approach it from a themer's perspective and figure out _how_ he want to shuffle form items around. Then we can worry about the implementation details.
3) People seem to like arrays more than strings. ;)
That's not an argument. -- Dries Buytaert :: http://www.buytaert.net/