It makes perfect sence to add a tag to a question that the tag item was the problem.
It does not make so much sence to add a tag to the question that the tag item is the solution. Unless the solution was the method requested in the question.
While the solution today might be obvious and done only one way today, If tomorrow a much better solution exists. The solution does not require the method, or some rediculous and extensive method to solution the problem, then the tag itself , the method of the question, will reduce the usefullness of a solution that no longer requires the difficult method.
If someone already (magicly) knew of an answer that did not require a specific method , or provides any Different method. For the times when the first method just doesn't work for everyone the same way. The question is now tagged for a method, and that would be very limiting to answers , and general learning new methods.
A Fake Example: User asks how to rearrange some element of the os, and the solution is to script, or hack a Dll, the tags script and hack are added to the question, as if the questioner wanted to hack and script. Tomorrow the element of the os is added to the adjustment ability, or a good program adds that element without scripting and hacking. Now the answer looks pretty stupid , and the question has limitations created by the tag.
Of course you know what is and what isn't, and what will never be, I am just discussing the premise of adding answers in the form of tags generally.
If the intent is to re-draw the questions target, after shooting out an answer that misses the target of the question completly. Then go write a How-To or a blog instead and point people to it :-) That does still make a good and complete and full answer IMO.
Answers that don't use a method that the Questioner wants to, or feels capable of appling, are still GREAT answers, and still valuable and usefull information to everyone. but Questions should not be Re-Molded and Edited , and re-Tagged to get a Question to meet the answers. Not only is this conforming a question to an answer, How does the next person asking the question similarly WRONGLY (even) find the valuable answers, when the poor way the question is asked no longer exists in search?
If the "stupid" question, gets a Good answer that changes the question or the questioners mind about what they even asked to begin with, that is great, but at no time should the question then be limited to the Method by a re-write.