Many people only used the ODD for OS and software installation and media consumption. (Either playing a CD or DVD, or ripping it to your machine, for personal playback, of course.)
It's getting easier and easier to install the OS from a USB stick now, with retail IE10 only coming in either a download or USB stick, no ODD. Music distribution is steadily moving towards online distribution, with movies being the same. Then add the availability of quality (and fast) external USB powered ODDs ...
Internal ODDs are becoming less of a 'thing' now since there are so many options that start to negate the need.
That said, the internal volume that would be taken by an ODD, even a slimline version is being seen as free-space to either utilize in an alternate way, or get rid of entirely.