Regarding mastering- I think it can help a recording, although I wouldn't say it's an absolute necessity either. A lot of great records have benefitted greatly from being mastered by a great engineer- mine, included. The fact is, a badly mastered recording can also lose everything that was great about the final mix.
I've encountered mastering engineers who will add massive amounts of compression to a mix, just because this is what everyone else in the world of professional recording does. Some people really like the effect of excessive amounts of compression on an already compressed mix, but this also alters the structural integrity of the mix. In some cases, I've actually felt that having a mastered version which sounds almost identical to the final mix is preferable to a version which has all those bells and whistles.
Working with a mastering engineer can be helpful- it can also be subjective. I guess it helps to know the engineer and what he will do with your mixes.
I've wondered what some one who makes records with such sonic integrity (such as yourself) felt about the process. Admittedly, there are a number of recordings that I would love to hear in the pre-"mastered" stage.