The Tagging metaphor refers to the use of single value keywords/attributes to label or tag individual items - as used in folksonomies like flickr and In WikidPad this could be done by adding a attribute pair. For example adding [tag:programming] to any wikipage that has anything to do with 'programming'

is there space for tags in wikis?

The use of attribute/keyword tagging of pages might not be completely necessary, as wikis already allow for similar lookup functionality using CategoryPages?, not to mention simple looking up of parent and children pages. However, the use of the tag within the wiki might yet add value, as long as the use of this metaphor does not undermine the wiki-way of rapid and ubiquitous linking to find material in the wiki - just maybe these two metaphors can mix as separate views to the same wiki data.

what value does it add?

For starters, the ability to find pages by multiple arbitrary tags - so, one could find all pages tagged with both 'programming' and 'friends' for a list of pages related to both, without needing those pages to be linked into a special FriendsWhoAreDevelopers page. (okay, maybe thats not the best example, but it'll do for now - JN)

The use of intersections of tags to find stuff could be extended with a set of TagExpressions, allowing for intersections, joins, and complements

This advantage of tags could possibly be carried over to the CategoryPages? metaphor by allowing the user to FindPagesByMultipleReferrals - so that one could find all pages that link to both CategoryProgramming and CategoryFriends

check out TagView for plugins to help tagging in wikidpad