Konstantin Obenland has been involved with the development of the last three default WordPress themes. In this post, he makes a great case for simplifying the default theme with Twenty Fifteen; and he goes as far as advocating simply using _s as is, only adding a custom
style.css, a la CSS Zen Garden.
His argument is compelling, and I tend to agree with the call for greater simplicity. I'm not sure how I feel about limiting the theme to only changing the stylesheet, though I love the competition aspect that's possible with that.
Also, read Lance and Ian's comments. Both of them also have significant involvement in the default themes (and have each had a huge impact on WordPress theming in general), and their comments make great points, especially in regard to the relationship between the default theme and core development.