In reflection of recent shenanigans on superuser, I was wondering if it would be worthwhile to mandate new users (Under 15 rep or something low) not be able to post new answers repetitively. Make them wait 10 or 15 minutes.
This site is designed to encourage new users. We want them active and helping as much as possible. If a new user finds a question that they like and they answer it, it is completely reasonable that they may find another question and answer that too. The last thing that we want is to say "you're too new for us to want your answer" because their response would be "Okay, I'm navigating away". Remember that the users are offering their own free time. The last thing we want to do is reject it.
As for spammers, if we shut them down from posting multiple answers, they'll create multiple users. If we shut them down from creating multiple users, they'll disguise their IPs. That's why we have spam-detecting algorithms.
Every new barrier to entry damages the site usability. Don't Do It.
In response to Josh K mentioning the 20 minute limit on questions:
There is a fundamental difference between questions and answers. Receiving an answer is something to be acted upon. So, if a user has more than one question in twenty minutes, it is reasonable to assume that they did not give enough time and effort into their first question. Or rather, if they are completely separate questions, perhaps they should focus on solving the first one before they move on and ask their second. This delay does not exist for answers. Once you have answered a question for a user, there is nothing more to do. It is hopeful that they will move on and try to answer another one, we don't want them logging out, or hovering over the one answer they are allowed to give, in the hopes that the OP posts a response or comment.
Answers are something you deliver. Questions are something you engage in. Therefore, timelimit on questions is reasonable, timelimit on answers is not.