Good
As it has been mentioned multiple times before, it is a good practice to prefix all column names to easily see which column belongs to which table - and aliases are shorter than full table names so the query is easier to read and thus understand. If you use a good aliasing scheme of course.
And if you create or read the code of an application, which uses externally stored or dynamically generated table names, then without aliases it is really hard to tell at the first glance what all those "%s"es or other placeholders stand for. It is not an extreme case, for example many web apps allow to customize the table name prefix at installation time.
