Merge-Delete Playlists v3.0 (formerly named "Batch-Delete Playlists") will allow you to merge the track contents of two or more playlists or delete any number of playlists at once, including Smart, Genius, and folder playlists. The merge feature will prevent the same tracks that may appear in different source playlists from being duplicated and has an option to delete original playlists. The delete feature only deletes the playlist; tracks, of course, remain in the library.
Very handy when assigned a keyboard shortcut.
This latest version is written as a Cocoa-AppleScript applet for OS X 10.6 or better; adds the merge feature.