Here is another example where useful information is deleted by Gilles. (I was going to refer a user on cstheory to the answer posted here but I can't since it is deleted, and it took me considerable time to find even the deleted post.) Moreover since it is deleted by a moderator other users cannot vote to undelete it.
Let me emphasis that this is not about deleting closed question per se, it is about unilateral deletion by moderators by default without consulting the community.
The arguments for justifying unilateral deletion provided so far are very unconvincing.
If users see a lot of closed questions, they’ll note that we don’t enforce the guidelines, so why should they? Without any final resolution, asking questions that get closed becomes something we are implicitly encouraging — a broken windows problem. If this goes on for long enough, we’re no longer a community of programmers who ask and answer programming questions, we’re a community of random people discussing.. whatever. That’s toxic.
This is a very illogical argument if by guidelines we mean "deletion": to put a bad guideline and then say that we have to enforce it because otherwise the guidelines are not enforced is absurd. If the guideline means not permitting off-topic questions, closure is a sufficient demonstration of that, it is also information for the user who might want to ask similar question, not only it shows that similar questions are not welcome, it tells them why they are not welcome.
If enough of these closed questions are allowed to hang around, they become clutter that reduces the overall signal to noise ratio — which further reduces confidence in the system.
The deletion only occurs after the question have left the front page, so this is not a real issue there. When searching, a user can search for non-closed questions easily using "closed:0" in their search so this is not a problem there either. It is like going over Internet and removing sites that we don't like so the Internet is more "clean".
Another argument which is expressed implicitly is that this is the general rule of SE sites. This is not the case as the Math.SE demonstrates. The guidelines on MSO are generally based on the situation on the trilogy (SO/SU/SF) and may not need to be required or even beneficial to other sites like CS.SE. My perception based, on the previous meta discussions, is that we agree that CS.SE is closer in nature to sites like Math.SE than to SO or cstheory, so if we are going to adopt policies from other sites, then it is more natural to check the policies on Math.SE than on SO.
Here are some arguments for not deleting upvoted posts:
These posts can and do contain useful information (as illustrated by votes), so they make the Internet a better place,
Authors have put effort to write them, it is disrespectful to them to unilaterally delete their posts, (Should I spend my time writing answers if a moderator is going to unilaterally delete it although there isn't anything wrong with my answer?)
No one (other than the moderator) has indicated a problem with them staying on the site, if the post is really harmful to stay on the site then it shouldn't be difficult to convince others users about the deletion.
This automatic deletion of closed posts seems more to be about the personal taste of the moderators and than a required action to help the site.
Since this has resulted in confusion, let me repeat once again that this policy is only about unilateral deletion of upvoted posts by default. The existence of similar, larger, and very successful SE sites like Math.SE where closed questions are not generally deleted puts serious doubt about a real need for this automatic deletion of all closed questions. If a post or a type of post is harmful in general then it shouldn't be difficult to find 5 users to delete the question.
Unilateral deletion of upvoted posts by binding moderator powers should be limited to extreme cases where the community cannot act in time to deal with the issue and should not be a general rule, not highly upvoted posts without any user other than moderators indicating a need for deletion.