It appears the Dolby PLII is mainly meant for improving PL's music performance and not so much for Movies or TV. Nice to have if you are buying a new processor, but not worth upgrading to on its own.
http://www.hometheaterhifi.com/volume_8_1/dolby-prologic2-3-2001.html
"Introduced last year, Dolby Pro Logic II is an improved, more intelligent matrix decoder. So improved is its decoding, that it succeeds where Pro Logic performed poorly, i.e., playback of non-encoded material such as conventional 2-channel music. That's not to say it is a music-only application, but it certainly makes for a new and wide appeal. Conventional Pro Logic decoding lends itself perfectly to motion picture soundtracks."
http://www.hometheaterhifi.com/volume_8_1/dolby-prologic2-3-2001.html