I believe the main reason xmonad-extras is not part of xmonad-contrib was something
related to dependencies the authors did not want to force on everyone.
Regex libraries, XMonad.Prompt.Eval (although that may be off in its own library at this point?), etc., yes. Note that a compatible hint library on hackage for X.P.Eval actually delayed the release of xmonad-extras last time around, so this isn't theoretical.