Please do not take my answer as a critique of any regulars here
The Mathematica site has a problem that is a good problem to have: we have some really expert, really dedicated users who can answer a wide range of questions very well. And these experts live right around the globe in many timezones, so they can answer just about any time. This is very good for people with questions.
But it has a side effect that some might not see as so positive: it is hard for less experienced users (or those of us who can't -- or shouldn't -- devote much time to the site) to find a question they can answer, that hasn't already been answered really well. This will be a particular issue for those who learn by trying to find out the answer for themselves: speaking for myself, I am a much better Mathematica programmer since I started participating on this site and its StackOverflow tag predecessor.
I have personally posted answers and then revised them quickly. Usually it's because I thought of something else to add rather than deliberately putting a stake in the ground. But the knowledge that there are fast-posting experts has definitely often added to my haste in clicking the Post Your Answer button.
This is an inevitable issue as the site grows and attracts new people who would like to be regulars. I think the fact that mid-range-reputation users have posted answers to this question has shown how keen some relatively new users are. The regulars in the SO mathematica tag need to be mindful that we are growing a community here, and new members want to feel that they are contributing. This does not mean that high-rep users should hold off from posting, but breathing for five minutes every so often won't hurt.
Some final, short observations designed to encourage people to post answers anyway.
- The existence of long and highly upvoted answers has not stopped some newer users from posting a subsequent answer to the same question (recent example). Indeed, sometimes it is worth doing so even if there is an accepted answer already.
- For the simple questions, there is often more than one way to do things, so more answers are fine.
- StackOverflow is actually designed around getting multiple answers. (Area 51 recommends an average of at least 2.5 per question.)
- Even if your answer isn't accepted, it will probably still attract some upvotes.
- Posting a duplicate inadvertently isn't a crime, and the later one might actually be explained better. You can always delete afterwards if necessary.
- You can gain reputation by posting some really awesome questions, too.
Conclusion: while there is no harm in posting a partial answer and then editing, there is no great benefit to everyone else in doing so. The rest of us should remember that we shouldn't hold off from posting answers even if someone else has already done so, unless they really are duplicates.
Addendum: I just wanted to note an important point exemplified by this recent post. b.gatessucks's answer was first, correct and accepted. But by providing a more extensive answer that explained in more detail why things work that way, my later answer currently has a couple more votes and the OP found it very useful. Personally, this is my favorite kind of answer to write. The question was quite simple and many of us could quickly come up with a drive-by solution, but I find building understanding to be more rewarding, regardless of the rep points.