These are good questions, Kent.  I must say I was a little surprised when I
bought your incredible book to see that it contained so many plot
summaries.  Personally, I find it more useful (generally) to get the kind
of overview of criticism and critical evaluation that is in the MT
Encyclopedia (and even though the encyclopedia is uneven, some of the very
best MT scholars are in there, and their articles on various works are
often excellent).  I also appreciate the bibliographies they give for
individual works.  That said, it's been useful to me to read the plot
summary of a short work to remind me of something, and especially useful to
find something in a longer work ("Now just where is that little story in _A
Tramp Abroad_?").  And as to students, Kent is right that there are already
plot summaries out there--not only Cliff's and Monarch's, but also in
reference books like McGill's in the library.  That's usually only for
major works, so Kent's summaries of the shorter and lesser works are the
only things out there.

Myself, I don't worry too much about students relying on plot summaires.
The first and only time I resorted to one (might have been Moby-Dick in the
11th grade), I found them so boring and I retained so little that I went
back to the novel.  I don't use quizzes in my teaching, so I don't worry
about that.  (I have students write response questions either in or out of
class, questions that would be difficult or impossible to answer from plot
summaries.)

BTW, please don't misconstrue my comments above as any disparagement of
Kent's excellent book.  I find his achievement incredible!  I was just
saying that on individual works, I usually find the MT Encyclopedia more
useful than MT A-Z for what I want.  But on many other topics, I find
Kent's book better.  I'm glad I have both, and I'm really glad both exist.