First of all, tag wikis are not meant to be alternative Wikipedias.
The full tag wiki is a detailed introduction to the topic, suitable as a destination for those curious about it:
what questions should have this tag?
some basic definitions
brief introduction to the subject
important links for learning more
one reasonably sized page
(source: a sidebar widget shown when creating a tag wiki)
Some of these points can (and have been) copied over from Stack Overflow tag wikis to other technology-oriented Stack Exchange websites, and risk becoming outdated. But that's a problem for the original tag wikis as well (in fact, all quoted content, also in answers, suffers from this problem); therefore, users should try to create something useful to the specific community. One of the reasons to reject tag wiki edits mentions this as well:
Generic descriptions such as encyclopedia articles and ad copy do not provide useful guidance; try creating something useful to this community specifically.
If you see content in a tag wiki that is blatantly outdated, feel free to update it, but you might run into reviewers who'll reject your edit because of the reason mentioned above. Also, don't expect Stack Overflow inc. to build something which allows for nesting content in tag wikis so that only the original needs to be updated.