Model and Form

Its because you have then clean data in the mvc philosophy as discussed also here:

http://forum.codeigniter.com/thread-29.html

In the model in a web context normaly you get the data and nothing else. Thats why you forced to do all the needed business logic in the controller (like get the input data).

But it depends on you if you want programm in your project in a clear mvc way or in your own way. but you should go with mvc ;o)