The reason USB loaders support the ability to rip Wii backups is because typical DVD drives lack the capacity to rip retail Wii discs. However, with the right software, you shouldn't have a problem ripping DVD movies using your computer. DVD playback alone isn't very stable on the Wii, let alone DVD ripping. While it lacks the ability for DVD ripping, I recommend you use WiiMC on your Wii for DVD and video file playback.
On a slightly related note, I recommend ripping DVDs using your computer so that you're able to convert them to something like Xvid; otherwise, your DVD rips will take up an obscene amount of room on your hard drive.