Con: answers that don't contribute anything to Ask Ubuntu, or to Stack Exchange in general. This site is not a forum.
If a question is a possible duplicate, you have two options:
- flag it as a duplicate
- provide an answer that covers the case where it is not a duplicate
Nothing prevents you from doing both.
To paraphrase Dolores Umbridge:
Answering for answering's sake must be discouraged.
If you don't know it's a duplicate, please, go ahead and answer. Perhaps your answer will contribute something new, in which case it could be merged into the original. If, in the process of answering, you find that it is a duplicate, but feel you can add something, continue to post your answer (preferably in the original).
Also see this Meta Stack Exchange FAQ: How should duplicate questions be handled?How should duplicate questions be handled?