Most of the time (?) a following dummy noun is required when the speaker wants to put a 用言 where a 体言 is expected, as shown by the sentence below:
However, it seems that sometimes a 用言 on its own can function as a 体言, namely, with no dummy noun added, such as:
So the question boils down to:
When a 用言 is used as a 体言, in what case is the addition of a dummy noun 1) required, 2) optional, 3) plainly wrong?
All three instances are from the novel Botchan.