Personally I think that the SD card slot is very useful on the K1 and would be even more so on the K2, especially now that it can handle pdfs (though the pdf handling still leaves a lot to be desired in my opinion, but that's a different issue).
Sure you could add more ram to hold more files--but you can only do that when the reader is constructed, and once the RAM is full it's full. Add an SD card slot, and you can have 2, 3, 4x more "RAM" just by tucking another SD card into your wallet.
There's nothing wrong with using the Kindle as a backpack, holding a handful of books you plan to read soon, rather than as a library. But I'd like to at least have the option of using the K2 as a library.