I just picked this up from MSDN forums on SharePoint development: Martin Hatch found a great fix to the issue of fields in content types.

Without this code, you have to define fields from content types in the list's schema.xml definitions, which is a HUGE pain, and leads to maintenance problems. This is because lists define the entire schema for the list, and content types are data objects that can be defined within that data space, with the potential for several content types within a single list. But having to define the data schema in both the list definition and the content type definition is a major drag, and kills the whole concept of schema reuse.

Check out Martin's fix for yourself-- a definite best-practice going forward, and something I'll be sure to include in as an "addendum" if I ever get around to such a thing!  Here's the link: [Code Update] SDK and Microsoft Press Both Wrong?? Custom Fields in the schema.xml