Other drawbacks of the ADT:
* When you need to "change" the type the effort is more than with native
* When dealing with temporary tables you have to have them in model by the time you restart SQL Server or use a startup procedure to add them to tempdb. This is harder than native datatypes
* replication has issues with some of these types
* Because it hides implemetation details it forces you to find out the definition.
* Like posted above not many providers deal with this successfully
* Linked servers can be trashed because of this
Because of all the above (and possibly more that I may be missing) I would vote for the use of ADT as a BAD parctice!