Authoring on TechNet Wiki can be fun because you can write about anything Microsoft related, including Microsoft Forums:
I created this list of pros and cons...
1 - You want the asker to know that you're trying to answer the question. So you want your suggestion to be seriously considered and tried ASAP (versus having the asker not try it and keep getting frustrated).
2 - Nobody else is proposing your answer. You replied to try to answer it. Time has gone by, nobody proposed your answer, nobody replied to it and commented on it, and nobody marked it as an answer. Rather than feel like you wasted your time (and possibly give up on the forums), you propose it as an answer to try to get a moderator or the asker to notice it and thus mark it as the answer.
3 - You don't want to leave it unanswered. Similar to #2, you tried answering the question and nobody is proposing your answer, marking it as an answer, offering an additional answer, or offering more details about why your answer doesn't apply. The Asker has obviously moved on, and you don't want it in the list of unanswered questions when you obviously tried to answer it. So you mark it as a proposed answer so that you can focus on the questions that are still unanswered.
4 - Your post got moved (especially into the "Off Topic" forum). If you tried to answer a question, and your post got moved (such as into the Off Topic forum), then you might want to propose it as an answer just because that makes those posts (that actually have an answer in that forum) easier to find... and thus are more likely candidates to ever be moved into the correct forum.
1- You don't want others to not look at the question. As Adavesh mentioned here , people who are trying to answer questions might skip it because they think it might have already been answered. However, as a counter step, the Asker, Moderator, or Proposer, can unpropose a proposed answer. So if the Asker really cares, they can clear the status so that this isn't an issue.
2 - You don't get Achievement credit and medals for it. I wouldn't make it a habit of self proposing. As in Naomi's example , these two people who self proposed (http://social.msdn.microsoft.com/Forums/en-US/transactsql/thread/e454b2c4-04c6-4d5a-8c5b-8920866ee7c4 ) have made it so that they don't get credit toward a "Proposed Answerer" achievement. For example, for the Proposed Answerer 1 Bronze Achievement medal, they need to be proposed by another community member 1 time. They'll never get the medal if they propose their own answers. (Granted that this might not be a big deal to many people.)
3 - The community doesn't really like it. It's obvious from conversations like this forum post (http://social.microsoft.com/Forums/en-US/Profile/thread/01fe255a-c18c-489d-86e8-7fbe88630c3e/ ) that you shouldn't constantly self propose simply because lots of people dislike the behavior. Generally you want the community to favor your behavior. Because of this and the above reasons, I would limit self-proposing to the conditions set above in the "Reasons to self propose" section.
Naomi and Luigi also gave their opinions:
Naomi N: My thoughts: Even though the reasons for self-proposing look solid, it still doesn't feel OK to self-propose. In other words, you have to be bold enough for self-proposing answers (even then you're absolutely certain it was an answer).
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Also, do you have any additional pros or cons? Add those too!