I would argue that being limited to two levels is a feature.

Here's a related quote[0] from Edward Tufte.

"Dr Spock's Baby Care is a best-selling owner's manual for the most complicated 'product' imaginable -- and it only has two levels of headings. You people have 8 levels of hierarchy and I haven't even stopped counting yet. No wonder you think it's complicated."

[0] http://web.archive.org/web/20080610092329/http://blogs.sun.c...