Do you need one??
The only real use I can see for having one of these is if you are making full length feature films with multi language dubbing and subtitles, director's cuts and all the other extraneous stuff that fills commercial DVDs - or making less than legitimate copies of course!
As a relative newcomer to DVD preparation for commercial use, I cannot see why I would want one of these - a 4.7 GB is quite large enough capacity for anything I could want to do. Things change, of course, and I may be wondering however could I manage without one in a few months time.