Dual card slots with Dual Writes also means instant backups don't forget. Ever have a card fail? It is rare, and it would suck, but it does happen. Having two copies, each on a different card, makes it far less likely to have data lost.
In theory dual slots also gives you the option of alternating writes between two cards. Shoot a long stream of images? First image gets written to Card 1, Second to Card 2, third to Card 1, Fourth to Card 2, etc... With each card on a dedicated write circuit you have basically doubled your write speed, clearing your buffer that much faster. (I have no idea if Canon actually supports this in any existing dual card body)
Personally I really would only want dual cards if they were matched, and I would only buy cameras that support CF, or a new generation of CF like memory. I would use a CF-SD if it was there, but not a feature I would push for.
Canon EOS 7D | EF 28 f/1.8 | EF 85 f/1.8 | EF 70-200 f/4L | EF-S 17-55 | Sigma 150-500
Flickr: Real-Luckless