The movie DVDs you have are all dual-layer, which means they have twice the capacity as there are two layers of data inside of the DVD. Recordable DVDs, however, have been single-layer. While the recordable DVDs are great for backing up files, the smaller capacity means that you couldn't backup movies onto them. Also, for people like myself, 4.7GB isn't enough space to backup my digital camera photos, and I don't even think about backing up my music collection.
All that can change now that Philips has figured out how to make dual-layer recordable DVDs that will eventually burn at 16x. I'm sure the discs will cost twice as much, but it would be worth the convenience.
- New Double-Layer DVD R From Philips