I prefer that a relatively well worded but completely wrong answer remain but be downvoted so that future readers know that it is a Bad Thing. The more comments on the answer that explain why it's bad, the better.
I only flag answers that are:
- Obviously shilling for a product.
- Completely unintelligible.
- "Me too!"
- Asking a question of the poster.
- And sometimes: is so hideously formatted that it would take more than 15 minutes to correct.
Keeping bad answers around for posterity is a Good Thing.