I know there is an "official" StackExchange meta-post already concerning this question, however I think it would be suitable to have our own post that reflects our position.
This really excellent question has provoked a number of excellent answers, one of which was self-deleted after the OP requested the answerer combine both answers into one.
In my humble opinion, this reduces the effectiveness of both the answers on this question, and the site in totality. It reduces the effectiveness of the answers on the question since you can neither upvote both answers (what would be two votes), nor can you upvote only the answer you like. Similarly, you cannot downvote one of the answers, if it appears to be incorrect or not very "good". It reduces the effectiveness of the site by not allowing the specific best answer to percolate to the top via upvotes.