Is it bad style if a section has exactly one subsection?
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, …