I'll chime in... as devil's advocate...
i'm an analog nut job, hater of the loudness war,
and believe in the old school of analog tape and desk, mixing and mastering.
However, things change. I believe the mastering process is destined to become part of the mix buss ITB (in the box or computer only mixing) for the professional engineer.
For other media, such as vinyl or tape conversions, you will of course need special tools and skill set, which usually incorporates a third party mastering engineer.
Now any quality engineer has all the tools available at their disposal that "pros" use, as well as the readily available knowledge to negate the extra process as they mix.
Mastering was created to make up for short comings in mixes, and to create a consistent product in the medium of the time (vinyl and tape).
We are dealing now with a media that is more consistent (digital) and is a platform where the tools ITB are financially in the grasp of everyone more than ever. A few thousand bucks and you have what the pros use- not a hundred thousand like the old days. (A few grand is nothing to sneeze at for most, but we are talking about professional mixing here).
Every little tracking house in my area offers mastering,
so that is proof enough to me that those tools are already pretty much in place, and fast becoming a standard skill set.
Having a third party evaluate, etc. is of course nice but why add another financial step to the process if you can do those things right the first time? Ideally a mix will be so good, a mastering engineer won't need to do anything, correct?