iBooks doesn't come installed on iOS devices, so that's not what happens when you use the device as it comes. (You just get the ability to Quick View it.)
I think it's a little strange to have to download an application called iBooks so that you can save documents (that you need to email yourself). Apple tries to be very intuitive, but document management on iOS is a mess. There is much room for improvement. I've had multiple people ask how to get documents on their iPad, which stands out because most everything else is so simple (especially after iOS 5 not needing as much of an iTunes lifeline).
On Kindle you go to Docs and it says "Nothing here, email something to XXX@kindle.com". You try that and boom it shows up. If you want to load some documents on your iOS device there's no obvious way without getting extra apps (which itself isn't obvious). iOS 4 addressed this somewhat to allow management through iTunes, but even after plugging it in it takes knowing to go to your device in iTunes, selecting Apps, scrolling down, clicking on an App under "File Sharing", clicking the "Add" button and choosing your file (then sync and hope it works, it doesn't let you know what file types are allowed so it's a crapshoot).
It's a side effect of iOS not having a shared repository of files. I'm surprised that iCloud didn't attempt to fix this, but it only keeps application specific documents in sync. Dropbox is still king, but because they're not integrated into the OS it feels like a hack. Maybe it's coming, but right now the iCloud folder on your Mac is hidden: http://www.macstories.net/tutorials/use-mobile-documents-fol...
Fair enough, though I'd suggest that iBooks' position as the Apple book reader will mean that most people who download apps will have it (though I know there's a sizable amount of users who never download anything).
Maybe not "as it comes" but even if you dont have iBooks, the "Open in.." menu will show any app that support the filetype. I can open pdf email attachments in iBooks, GoodReader, Dropbox or TabToolKit.
Because if you don't have an application to view it with outside email, what are you going to view it with outside email?
iOS is moving away from the user-facing file system. Normal users don't get it; they associate content with tools to process it with, not as content just sitting there wholly independent. If there isn't something to view the content with, the notion of storing content to not view is nonsensical.
I don't think people have difficulty with files; some fraction of people have difficulty with hierarchies of folders, but files? I don't think the evidence is there.
selects 'open in iBooks'
Seems to work here!