Is it bad style if a section has exactly one subsection? In a discussion on why subsubsections are evil, egreg comments:


  If you have subsubsections, you must have at least two of them in a subsection; you must have at least two subsections and, of course, at least two sections. Draw the tree and you'll understand.


When I'm writing long texts, such as for my thesis, I might have sections that contain only one subsection.  For example:

5.3 Topic X

... bla intro bla ...

5.3.1 Special Case in Topic X

... bla detail bla ...


Is it considered bad style to have 5.3.1, but no 5.3.2?  Should I rather put the bla intro bla in a 5.3.1 Introduction and have the special case in 5.3.2?  What are the general approaches here, what is considered good or bad style, and why?
Having a section with a single subsection is usually considered bad style: most style guides, academic or not, advise against it. In particular, the Chicago Manual of Style advises against it.

Regarding the reasons why this is considered bad practice, I'll quote Wikipedia:


  Single Subsections
  Just as your English teacher told you, if section 2 has a subsection 2.1, there'd better be a section 2.2 as well. If you see a section with a single subsection, you have three choices:
  
  
  If there's a lot of text in the section, followed by the subsection, you ought to be able to carve out a good subsection from the initial material, or even two, to create multiple subsections.
  If most of the section's material is in the subsection, you may not need a subsection. Just combine the two.
  If the content of at the top of the section is short and substantially different from what's in the subsection, you might be able to promote the subsection (for example, change the heading from level 3 to level 2). On other hand, if the subsection covers something relatively unimportant, then don't promote it to a level 2 (top-level) heading.
  


Other sources, both style guides and writing advice, say the same thing: 1, 2, …