Tag Wikis define the tag for the purposes of the site, not the general topic. This topic was throughly covered in the blog post Redesigned Tags PageRedesigned Tags Page. Tag Wikis and exceprts are to teach people how and when to apply a tag. They are tag wikis, not topic wikis.
Stack Exchange is a Question and Answer site, and all features should be developed to support that, not as some weird, feature creep addition. Tag Wikis currently function to support the Q&A role of sites. Making them into topic summaries is not what they are for.
When included, topic summaries in Tag Wikis are included so you know when to use the tag. The tag wiki really doesn't need to be a starting off point to start learning a language or topic. There are hundreds of sites for that, but a Q&A site is not one of them.
As pointed out in this postthis post, putting subjective information in tag wikis is the worst possible place to do it. Tag wikis can't be closed or voted on, if tag wikis become reading lists or fun facts or any irrelevant junk like that, they're useless. They are meant to help understand how a tag should be applied on your site, not be a Gentleman's Club of neat tricks High Rep Johnny found out and decided to slap into a tag wiki.