PDFs are easily saved by selecting "open in iBooks". Or, you can bookmark a PDF by URL.
Because that is extremely intuitive. I can see Apple patenting this right away.
Either you think that users should be able to save any files they want to their devices (and move them to other devices), or you don't.
Personally, I wouldn't accept this even if iBooks was preinstalled. There are so many other file types that you just cannot move/save.