I don't know that it needs to be a fully functional computer but I do agree that we need some sort of file system that you can access. In other words, while I am typing an e-mail I would much rather hit the universal symbol for "attach" than to figure out halfway through my e-mail that I wanted to attach an image, for example. This is something that I can do on my Droid X; I can even upload an image from the DX here on the forums using the image manager. There are shortcomings of the DX, some things that aren't as smooth as the iOS platform, but overall this very "feature" is well worth it. It is something I've wanted on my iPad since I purchased it. I absolutely love my iPad, the battery life, the usability ... excluding the aforementioned missing "feature".
Attaching a photo to an email after starting it isn't something that requires access to the file system, it's just that this is the way that we're used to doing it. Apple could add a photo picker control similar to the one that Pages uses. Similarly uploading to photo hosting sites isn't something that necessarily requires access to a file system. Currently you can do this through Apps or email, and Apple could potentially add photo picker support to Safari for uploading photos through the browser.
I think the Library based approach that Apple are taking isn't just an accident of iOS's birth as a smartphone OS. Instead I think that Apple thinks that libraries of data contained in Apps is a good way to make computers easier to use for mainstream users. For example, it provides an answer to "Where is the letter that I wrote?"; on iOS it will be inside Pages (or whichever App you used to write it)
The downside of a Library system is that it is less flexible than a file system and hence more difficult to use for those people who understand file systems. There are things that Apple could do to ease this without throwing out the Library system completely. A "Files" App that served as an interchange point would be one option for advanced users. I see this as serving a similar function to Goodreader, but with much deeper integration. Another possibility would be to enhance the ways that Apps can work together. For example make the picker control extensible so that the user can choose items from non-Apple Apps. Adding the ability to temporarily edit a file in an App, instead of always adding it to that App's library would also be useful.