Aaron Jorbin shared some bullet point thoughts on default themes and their role in the WordPress core development flow. I like them all, and you should check out what he has to say. But one resonated especially as both different and particularly enlightening:
Core learns from default themes. Nacin once said “Twenty Ten enabled the core team to become theme developers once again”. This is needed. Most people on the core team that build themes are building pretty advanced themes. The needs of Conde Nast, The New York Times and most of the clients of 10up, XWP, Human Made, etc are not the same as the needs of people who install a theme from the WordPress repository.
This, and the rest, are very good points. Check 'em.