Having come late to this discussion, I don't see any good reason why the Internals of a UDF should affect the usage of the UDF. This to me seems like the Microsoft Developers couldn't solve their own problem so banned it instead.
Saying that it doesn't matter because you don't care is off topic. Yes UDFs do impact performance but sometimes there's no choice or sometimes the slower query finishes before the perfect SQL statement is found.
Regards
David