I don't get the purposes of the disciplined badge. Why would someone delete a good answer where he has been awarded 3 up-votes, and why should he be marked as "disciplined" for doing so?
It's disciplined because you delete the post even though you will lose rep by doing so - i.e. you actually care more about answer correctness and relevance than rep (or, as smackfu says, you care enough about badges that it pushes you over the line (gives you enough incentive to delete your answer, knowing you will get an extra badge out of it).
Here's a scenario I've been in many times: I post a quick answer to a question. It's accurate and reasonably helpful, and therefore collects several up-votes. Then someone else posts their answer... The extra time they spent on it clearly shows: it's comprehensive, easy to read, links to tangential-related information, includes good code samples...
And so I delete mine and up-vote the new answer. Maybe edit it a bit, to include something from mine that I feel would benefit it.
End result: the question gets a single, solid answer. The other guy is rewarded for the extra effort he put into it. I'm rewarded with... the satisfaction of a job well done.
And maybe a badge...
Here's my story. I've just deleted an answer of mine with 4 upvotes (while all other answers on the page were at 0 or 1) after the OP made grave edits to his question that rendered my answer totally asinine.
At worst, my answer now suggests that I just can't read. At best, it now looks like I mixed things up and posted an answer to a totally different question. Which, in point of fact, I have, but any new visitor to the page would have trouble to understand the situation. He would just see the question "how to do X", then my top answer "here's how to do Y", and conclude that I and those who upvoted me must be a bunch of morons. He would then probably proceed to downvote my answer (and rightfully so).
Of course, I could still just leave my answer sitting there, since it would take a whopping 21 downvotes to make it a net loss for me. However, not being considered a moron is well worth giving up 40 points IMHO.
Lesson learned: always keep an eye on the questions you recently answered. They can get edited in ways you'd never imagine.
Because supposedly if his answer got some up votes, which gets him reputation, but the answer is irrelevant, he is disciplined by doing the right thing and deleting it.
I deleted one after I misread the question and gave the wrong answer, but somehow got five upvotes anyway...
It incentivizes deleting an answer after it is no longer relevant. That's why I deleted the one that got me the badge.
On Stack Exchange, it's a given that earning rep, and (easy) badges helps reinforce positive behavior but it also seems to encourage a gamer mindset in some users. There is a reward at the end of a task, “you” see the reward as a means to an end. The task itself is unimportant, it is the reward/prize/points earned etc. that counts.
Do not treat Stack Exchange as a place to accrue points or badges in order to show or validate your self-worth, therein lies madness. This is a Q&A website. Ask or answer a question to the very best of your ability. Be meticulous. Be exacting. Be accurate. But above all, be honest. Consequently, armed with this new standard, you might find your upvoted answer is misleading, inaccurate or false. If it belongs to the first two categories, fix the post. If it is the latter, delete it. You should be the best judge to realize whether an answer of yours was good or not.
In summary, the system rewards users who are honest about themselves and their mistakes.