I think this became a vice as WordPress grew in popularity.
The vice? Categoritis: placing an entry in a whole bunch of categories.
Sometimes I put an entry in two categories. But rarely: I try to stick with the one that seems most appropriate even if that seems a compromise. Once every year or two I may let an entry span four categories but those are special and very personal entries.
Categoritis may trigger Google’s (and those lagging behind other search engines’) duplicate content filter.
When Google sees the same content repeated across the web it tries to establish which copy is the most relevant. No reason to list the same words over and over again. In the past webmasters tried to create instant but worthless websites but copying all the content of DMOZ and the Wikipedia. This left the search engines ever more distrustful of repeated words.
Now Google really arches its brow when the repetition occurs within one site. Is the writer seeking to inflate his website’s page count the lazy way?
And when for each category you place an entry in you create effectively a duplicate page because the entry will appear in the archive for each category.
Google may think you a wicked person and penalize all of your pages.
I’m not saying this will happen. Exactly how the search engines respond to this isn’t clear.
But I notice those who probably know the most don’t commit the sin of categoritis.