Why is there not implemented in \subsection a \leavevmode at the end of it? Here a minimal example showing that starting with the first word in an \hbox encounters vertical mode. Thus requiring a \leavevmode to put it on line with the following text. Whereas not starting with an \hbox does an implicite switch to horizontal mode.
I can imagine that this behaviour could be confusing for users not well versed in the different behaviour of the horizontal and vertical modes.
Therefore my question: Wouldn't it be wise to implement that \leavevmode?