Because the two has so much in common...
But alright, how could they go wrong?
The only thing I can think of is the quickslots/favourites not being fast enough, but there were no problem with that before...
Honestly, you just mentioned my main and only real beef with dual wielding. If the favorites aren't fast enough or are difficult to work with, then it will get VERY tedious, VERY fast to play a battlemage/spellsword or whatever other hybrid would need to swap spells/shields/swords. In fact, it would get frustrating for ANY class. A warrior switching between dual wielding swords and to using a sword + shield might find it just as annoying as the most hardcoe of battlemages.
Now, I certainly don't think that they could fail with dual wielding the same way they did with Level Scaling, but every little feature counts, and things do add up.
I think Bethesda is a great company with hard working people, but they're only human. I expect flaws in Skyrim. Many of them. Just like I would with any video game. We can only hope that those flaws are as small as possible.