TwentyTen was designed to help understand the WordPress theming system and that is why it has more lines of comments then actual code. So looking at TwentyTen you need to understand that they tried to include every file in the Template Hierarchy (eg: attachment.php, single.php, page.php) and every template tag there is (including: get_template_part() which is relatively new).
The best practices would be to use as much theme files and template parts as you can if the theme you are designing / developing is to distributed to make the life of the people who use it as easy as possible in customizing it for there needs, but if its just for your own use then a single index.php with 3-4 template parts would be fine if you know what you are doing.
For example I’ve recently had a client that wanted to use EvoLve theme which is a beautifully designed but very badly developed. I needed to create custom templates for a custom post type and i can tell you that the it wasn’t nice at all and i had to do some major reverse engineering to get it done.
but then again it depends on what you like to do with the theme (share or self use) and even then its up to you.