Fluendo multimedia codecs exists as commercial products because of software patents (required for decoding mpeg, mp3, etc) and silly-DRM removal laws (ie parts of DMCA, Digital Economy and ACTA) that stop you removing the CSS DRM on DVDs. The Medibuntu variants are still free software as they're not infringing any copyright (to my knowledge anyway).
Whether you legally need it or not comes down to where you live honouring software patents or the DMCA-style DRM-cracking laws.
Regarding quality: the codebase for each implementation is different so there is scope for one implementation being better than another. The only test I've really seen is ffmpeg vs Google's own implementation, where ffmpeg trounces Google.
Dell plays it safe. They want top sell a product internationally that they can advertise as being able to play DVDs. That means they have to adhere to local laws and that undoubtedly means they'll need to license the software in some regions. I expect it is easier (and therefore cheaper) for them to just bulk-license from Fluendo.