Why can't I use newid() in a user-defined table-valued function? I recently found out that SQL Server 2005 doesn't allow the use of newid() within user-defined functions. Why is this?

I have an alternate solution that suits my needs, so I'm not looking for ways to get around this. I'm curious why the designers would make this decision.
SQL Server only allows deterministic functions to be used within user-defined functions. Since the value of NEWID() is not deterministic, it cannot be used. You will find the same thing is true with GETDATE() and any other non-deterministic function.

I'm not qualified to answer why they would make that decision. It annoys me as well but I'm sure the SQL team at MS are not slouches.

EDIT: It turns out that my knowledge about GETDATE() is out-of-date. As the commenter says, you can use GETDATE() within functions from SQL Server 2005 onward. However, you still cannot use NEWID(), which I believe has to do with the same non-determinism constraint.