The lack of an optical drive isn't a mistake, I don't think. By that logic, the ThinkPad X61 (one of the most popular ultraportables as well as one of the most praised) is also a collossal mistake. But the lack of an internal optical drive hasn't hurt it either.
The wireless system to borrow a drive I think is actually a nice option for people who don't need to do much with optical disks regularly; those who do and still want an MBP would still be fairly well served with the external drive (which looks pretty portable frankly compared to some other external drives I've seen).
But yes, if you really needed regular access to an internal optical drive, clearly it would not be the right option. But i don't think it's a big mistake any more than the ThinkPad X61's lack of an internal drive is a big mistake.
-Zadillo
I don't think it was a mistake to not include a built-in optical drive but I do think it was a mistake not to include the external drive.
What does someone do if their 1.8" disk crashes and they need to install a new system? How does one recover using a time machine backup file without some sort of optical drive?
Sure you can update OS X by connecting to another machine's optical but if you have a blank system disk you can't even connect to another machine without an OS. You will eventually need an external drive, so why not include it?
Also 5 hr battery life is horrible for a system with no optical drive. This thing should get 10 hrs to be competitive.