Architectural Question: Web Application + RESTful Web Services

[eluser]CodeIgniterNewbie[/eluser]
Would this be a good idea:

1. there is a model method foo() that the web app calls
2. there is a REST method foo(), which calls the model method foo()

In this scenario, the web app does not suffer any performance issues. I'm wondering if this approach can somehow become a maintenance nightmare, though.

Or is the difference truly negligible that it doesn't really matter?