I don't remember downvoting code-only answers, but I'm inclined to do so now.
The reason is that the context of why the answer works would be shared only between the original poster and the answerer. A person reading the code snippet later, may have no idea as to why the answer is valid. Ok, granted there will be snippets that will not fall under the previous definition, but that is not the point.
The point is that, the answer is useless to anyone not knowing the subject very well. This is as good as going on ServerFault and providing a shell script that has rm -rf /
to a novice with the adjoining line: "Run this!! This will solve your problem". Ok, most people would know that, but who would know drop table dual
in the context of Oracle. In plain and simple words, the likelihood of an answer being verified by multiple people reduces with code snippets that are undocumented.
PS: The answers of type "Look at this link. The answer is here." should also fall under this category.