See this (possibly now deleted) question:
The user just threw
abcde in the tags list, and thus saved themselves from entering a tag.
First off, that question doesn't look like a spam post. It just looks like a bad question.
Second, a bunch of people (especially new people) have no idea how tags work. This leads to interesting things like users typing
abcde or populating the tag field with completely useless things because they think they need all five tags.
That said, the
abcde tag is pretty uncommon - even on spam. It has a legitimate purpose, so there's no reason to nuke it or alter how it works.