|(English) This is an essay. It expresses the opinions and ideas of some Wikimedians but may not have wide support. This is not policy on Meta, but it may be a policy or guideline on other Wikimedia projects. Feel free to update this page as needed, or use the discussion page to propose major changes.
History is the main feature of MediaWiki: let's keep history, especially of our project pages (and even more WikiProjects) which document our trial and error.
The learning organization. But frankly, when three years afterwards I tried to understand what happened, and I need to be an admin to look in the deleted pages? Eh... I think it's illogical that for everything we try to develop on our projects to learn and to try to improve the situation and then it doesn't work we delete everything so that nobody knows about it and nobody finds anything? Let's try to keep all these things.
This could also arguably be said for articles: the short or under-developed articles and the controversial articles, two categories of articles which are not deleted because of the inherent or obvious inopportunity of their subject but because of their current condition and wording, could be moved to an incubator (subpage of some project page or special namespace) especially to give new users the time to understand (in the first case), but also to let future users read and understand the discussion (in the second case). See also ru.wiki experience with such an incubator, presented at Wikimania 2010.
Keep history for others
Keeping history is important not just for project internal purposes, but for preserving the history of the internet elsewhere. Wikipedia and associated projects have been around for decades and grew out of a thriving WikiCulture that influenced the development of the early internet. When Wikipedia was young and projects such as Ward's Wiki and MeatballWiki were still active, interwiki links were common and MediaWiki still supports linking to their archives (e.g., meatball:WikiPedia) just as it used to support linking to their live and editable pages. These archives are static, and when history is deleted their links break and cannot be fixed. This harms the archive by introducing en:Link rot and makes it harder to understand the early relationships between Wikipedia and historical wikis.