1.

There are a lot of artricles on the SQL Server Central website about undocumented features of the SQL Server and which were found by readers quite useful.

If there is a way to solve the big problem You currently have and there is no known documented solution for it than You will probably use undocumented one, even for the price of the problem in the future release of the product. Maybe it is not a good practice but when You are by the wall, You do not care.

If You do not approve it, You do not have to use it.

2.

You are right that any undocumented feature may change with the next version of the product but the same happens to some of documented features. They become "depreciated" and they will in all likelihood will not be available with the one of future releases (this will happen with extended stored procedure).

This is unavoidable , regardless of, that if You use documented or uncodumented feature (with that difference that undocumented ones may change with the new Service Pack).