Experienced mastering engineers offer a perspective based on working with a lot of albums. They can (and should) provide creative input, building on the musical vision of the production team. The result can be a new and different view into the music - an album that sounds "in the pocket".
In working with labels all over the world, phone, e-mail, fax and clear written notes are often essential to understanding the producer's direction. Still, there are times where the process does not work. The problem is not that the finished master doesn't meet high quality standards, but that the producer's vision is so specific that, in the end, they decide to master it themselves.
Fortunately, there is a common thread in most of these cases: the producer is also the writer and arranger, and often the principal performer and recording engineer as well. They are so close to the project that it becomes difficult for them to assign artistic control, even for fine tuning.
This is a fairly rare occurrence, since most music people appreciate the results of a high-quality collaboration.