I just think that a reply with any useful information content, whether eloquent or not, beats negative voting without feedback. And people who get down voted often are the people who would benefit most from some feedback.

Anyway.... One obvious reason for downvoting without, or in lieu of, an explanation is if someone else has already provided a good explanation. It's not like that magically exonerates the offending node. Massive downvoting could even help drive home the point (if the recipient is open to learning, or at least open to accepting/adapting to the way things are).

And I'm pretty sure that spammers will not be reading any explanations, but a certain number of downvotes are necessary before a node is reaped.

Regardless, your post is a good reminder that constructive criticism can be very beneficial, so thanks for asking.