I was quite pissed when Canon delivered their 5D MK III with the combo memory card slot setup. But the more and more I think about it. The more it makes sense to me.
- Backup. I went with the professional party line that dual CF was necessary in a pro body because you want a backup of your original card. Well, you can do that with a combo setup as well.
- Speed. CF cards are faster but that really only comes into play when you fill up your camera's internal buffer first. Even with my ancient but awesome 50D, that's a good 10 - 15 shots before that happens. Even the high-level pro sports shooters have told me they've never filled up their camera's internal buffer.
- Redundancy. I'm referring to needing different memory card formats. Yes, that's a bit of a bummer. It's certainly much simpler and easier to carry 4 CF cards that 2 CF an 2 SD. Totally. Or having to need 4 CF and 4 SD cards? Yes. It's a potential cost increase.
So I'm thinking the benefits of expansion and future-proofing with the SD slots to outweigh the negatives. Ideally, two CF slots AND an SD slot in a camera would be perfect in my opinion. But I don't see Canon doing that. With solid performance specs, you could keep that camera for a good deal longer. Hurting sales.