NO problem here. . .
I'm seriously thinking about it. My Prs350 doesn't have an SD card slot, and I get along just fine without it.
Also, don't forget the whole Google approach has really emphasized the the connectivity aspect of computing. I think Google's vision is that of a great display and calculating device with modest storage, connected to hard drives via the internet (or if you MUST, storage that Google can't control!). It's that whole 'cloud' thing. They are doing it with productivity, entertainment, you name it. And many of their free apps of this type are as good as the 'paid for' apps.
The approach for entertainment and other assets are stored on either Googles hard drives, or some other storage media and the inherent connectivity of the device and a good internet connection allows you to have virtually unlimited storage.
At first, I wasn't too enthusiastic over this approach, but I've been involved in SalesForce at work (A Cloud based Customer Relationship Management system - enhanced to be pretty much a complete cloud database development system). I think it works pretty well.
Also, we are already sort of doing it any way! Amazon is kind of doing it for my android phone and I assume the same for all reading devices. The purchased books seem to be in a 'library' somewhere on Amazon's servers. But the app in my Droid displays the books I 'own' (a pleasant fiction I can't seem to give up!). If I want to read something, I select it, and it opens almost immediately, it downloads the book as I start to read it. At any rate, the book is then actually residing in the 'library' and on the phone, when I am done with it, I can keep it on the phone, move it to my PC, or delete it at my discretion, since I am always free to download it again.
The need to store and organize books is much simpler for the casual reader, and not any worse for the well organized Calibre user. No, I'd say I am moderately pleased with this methodology and I no longer have to worry about space management on the display device so much.