runawayprisoner, there is a way to make a device USB compatible and still make it secure- aka put a password on the phone that needs to be entered before it will enter USB mode, hide system files or make them read only, etc.
If the system files are exposed, they will be read one way or another, and that's insecure in case you don't want sensitive information such as your contacts, text messages, emails, and photos, etc... to be available to anyone who has access to your device.
Requiring a password to enter USB mode is useless when the OS isn't running, or it's cumbersome when you have to restore the device and you can't enter the password. Plus it's not just USB but the dock connector is used for many other things, and you wouldn't want your phone to ask every time you connect something new.
Another problem is iOS pre-caches or indexes all of the files transferred over iTunes, so access to them is simultaneously without lag. If you were able to freely transfer files over, then the device has to scan your folders over and over again to look for compatible file types, which will undoubtedly slow down the operation and make it not as smooth as desired.
I don't think this is true. If you plug in an iPhone (or Touch) into Windows, you can drag and drop photos back and forth to a special section of the device. On my Windows 7, my iPhone's photo memory section shows up with as a Flash drive named "iPhone". If you can do photos, why can't you do MP3's or videos?
Because as I mentioned up there, if you can freely change the files inside the system, then the device has no way to know if something has been added or modified without scanning the files all over again. And file scanning is very slow on devices with limited I/O performance like the iPad, so scanning over files will make it lag more than necessary, especially more so when there are too many files.
Android did suffer tremendously from that. Especially Samsung's Galaxy S phones.
And that's even assuming it won't expose the security risks I mentioned.