[srett]

He was probably referring to the fact that preventing an overfull box without rearranging text is linebreaking too early, thus creating that excessive spacing. Also note that underfull boxes actually are a subjective thing; you cannot easily draw a line between "ok spacing" and "too wide". People will have different opinions on this, especially if they have no clue about typesetting they might not even realize it. So what (la)tex does here is quite clever in that it creates a layout that will make even the complete novice realize something is wrong. Of course then people can still ignore that but well, if you just don't care what your thesis looks like...