How much Scripture to preach on?

Is it really expository if you are going slowly through a book, or is it really thematic? How much Scripture to preach on? « Shored Fragments:

The reason for this is rather simple: there is an recent British tradition of ‘expository’ (meaning of scare quotes will become obvious) preaching, traceable back to Martyn Lloyd-Jones, although not much further, and presently reinforced by currently-popular neo-Reformed writings, that measures commitment to Scripture by, roughly, how slowly you preach through it. The practice of spending several years preaching through Romans or Philippians verse by verse-fragment demonstrates, it is held, a high view of Scripture because it is being taken seriously, mined slowly for all of its meaning.

I confess to being profoundly unimpressed by this argument