Situation: You’ve decided that you’re going to build a new computer. You put your part list together (case, mainboard, RAM, hard drive, processor, etc.) and then come to the point where you have to purchase an optical drive.
Seems like a waste of money, doesn’t it?
On the Linux side of things, it has been proven that you can install a full version (meaning not "biz-card") of a Linux distro with absolutely no optical drive whatsoever.
In addition to that the vast majority of Linux apps are download-only territory, so there literally is no need for the optical at all. For backups you just plug in an external drive via USB or Firewire.
On the Windows side you are required to have an optical drive, but only for one thing – the installation of the Windows OS itself. However you could easily get away with using an external optical drive, install your OS then put the drive away until you need it (which will most likely be never).
For Windows gaming there are download options, the best (arguably) of which is Steam. No discs necessary and tons (and I mean tons) of really great games.
Also bear in mind there is a large push by the computer industry to "go cloud" as you’ve heard, such as with Windows Live applications. All that stuff is from the internet and no optical drive is necessary at any time.
On your next PC box build, could you go optical-less? Yes, absolutely. If doing it the Linux way, you don’t need the optical drive at all. If doing it the Windows way, all you need is an external to get the OS install out of the way and that’s that.
And what to do with that empty bay? You’d be better off putting in a card reader where the optical would otherwise go. You’ll probably get more use out of it than you would with the optical.