I've been using functions in tables this way for many years, used with care and probably not in highly transactional tables the advantages are great. I have also used this functionality to simplify complex joins and selects to great effect. used with care you can also gain performance.

My view on sysmaint.exe is that it should be avaoided anyway - hey you're calling an external program out of process to run a dbcc ?

I see no problems to restrictions on changing functions used this way - in a production system you shouldn't be able to make an ad-hoc chnage anyway, so with proper testing what's the problem.