This is quite an interesting question. No other desktop applications depend on yelp, but they all use it as their help reader. Yelp is of course included in the default desktop as part of the ubuntu-desktop package.
The packagers seem to have taken the view that this is sufficient. Except for ekiga, apparently, see bug 78287 -- grip should depend on yelp https://launchpad.net/bugs/75188 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
