It seems to me that a third vote would be appropriate, in between the two
thumbs, and that it ought to be a question mark. Here's why:
1) The system presumes that members will vote, but it is obviously
the case that most post readings result in no vote at all.
2) When a member is engaged in the discussion at hand it might be rather
likely that he would not remember to click a thumb before proceeding, and
this may especially be true for exchanges where intense debate is going on
and strong disagreement, which ought to be registered as thumbs down
for disagreement, but the disagreement might be the very power of the
discussion, which makes the discussion a good one, and so no thumbs
might be voted at all. This is substantiated by the fact that the moderator
sees fit to allow certain members to continue and not to 'ban' them when
their presence adds an element of tension to the forum that is not
destructive overall.
3) Therefore, one or more members might be contributing a lot to the
overall content of discussions while they receive no thumb votes, for
there is no abiding benefit for any member to cast votes - there is no
counter (for the use of members) that keeps track of how many times
and/or how you voted. If any such register is accessible only by the
moderator, that means that members are merely increasing the mod's
authority and power over your contribution by letting him think he knows
more about what you are thinking, even if you are not specifically saying
it, in which case you're better off not thumbing anyone up or down,
but a third category would do no such thing against you and you could
well use that without any concern for being accountable, and that is
probably the biggest reason it wouldn't be added as a feature.
4) When a member wants to place a vote (which may be very rare) and
it is not an up vote nor a down vote, it is inadequate to say that not
voting at all is the same thing as voting "?" in the middle, because most
members do not vote at all when they read a given post, and a desire
to vote "?" is therefore lost among and indistinguishable from ambivalence.
5) The middle vote might be chosen by some members when they want
to say "undecided" since that is what you see on a lot of polls. But just
as the rules for thumbs are not understood (and therefore perhaps
erroneously used) by members until they are explained in a thread like
this one (and even then only by those members who read it!), so too,
could they read that the middle vote is for questioning why a post was
made at all, since it is off topic, or contributes nothing to the thread, or
has far too many typos and/or bad grammar to be comprehensible,
among other things, but is not so BAD that it warrants a dreaded down
thumb.
6) The tally of middle category votes could be useful for members to
see that they ought to pay better attention to the quality of their posts lest
they decline into down-thumb votes, or, that with a little effort, their posts
could become rated with up thumbs instead of "?"
These are the reasons I can think of off-hand for having a third category.
But, it might simply be a matter of the forum platform not allowing for a
third category if it's not an offered feature in the script.