I think the most important side-effect of using the Repository pattern vs. the ActiveRecord pattern is testing and extensibility.
Without having your ActiveRecord object contain a repository itself how would you isolate data retrieval in your test cases? You can't really fake or mock it easily.
Besides testing it would also be much more difficult to swap out data access technologies, for example from Linq-to-SQL to NHibernate or EntityFramework (though this doesn't happen often admittedly).
