One of the things that annoys (some) sysadmins is users exchanging files over the email system, often for good technical reasons, and over the years a lot of people have devoted a lot of effort to trying to persuade users to switch to various other mechanisms.

It's not going to happen. (Well, you knew that already if you've been paying attention.)

People keep exchanging files over email because email remains the most visibly secure and convenient way of doing so, especially to naive users. The advantages of email for this, from a user perspective, include:

• email does it all in one step; competing systems generally require both uploading the file and emailing the target, and then the target has to read their email and do something else to retrieve the file.

  (If you think that the service users upload files to can send the email itself instead of making the users do it, remember the first rule of free email-based services.)

• people naturally feel that it is more secure to do the electronic equivalent of mailing an actual package instead of the electronic equivalent of mailing directions to where you can get a copy of the package (no matter how obscure and unguessable the directions are, and no matter that in theory any of the postal people in the background could make their own copy).

• email is in practice 'more secure' in that your file is less likely to spread around, because people resend huge email messages much less frequently than they pass around URLs or the like.

• once you send the file, you don't have to worry about if and when it will expire; the recipient has their copy and that's just done.
• once you receive the file, you don't have to worry that you need to grab a copy before some set time or the like; you can keep it as long as the local policies on email let you (often 'forever'). Similarly you can delete it whenever you want to and it's gone.

Even a global filesystem with reliable identification and ACLs wouldn't get around the convenience of the expiry (non) issue, although it could get around the others.

And frankly, speaking as a sysadmin I am just as happy to not have another service to run, maintain, and worry about, so even for me all the alternatives to users emailing files to each other are pretty much worse. (And I would have to run the service, at least if we're being responsible.)