According to their presentation the OS is on the flash memory and the HDD is more for storage. Mountain Lion then automatically moves applications and files that you use the most to the flash side for faster performance, vice versa moves stuff you don't access often back to the HDD.
As for the optical drive, I understand why they are trying to guide the market away from that technology. In some ways it makes sense. DVDs are on the verge of becoming obsolete as more and more people get accustomed to streaming. (The head of Samsungs Blu-Ray division even said as much as soon as the format battle with HDDVD was over.) Not many people buy CDs anymore except audiophiles who play them on high end stereos any way. Same goes for software too. All of that content can be acquired via the Internet. And oh yeah, Apple has a whole ecosystem built for that (and if you are on PC, so does Amazon and google). USB flash drives and cloud services such as dropbox make burning discs for transporting data pointless.
The ONLY reason I personally would absolutely need one is when we have professional pictures done and the photographer gives us a disc. Then I upload the pictures and the disc goes in a box. On the other side of that coin if I were the photographer I would need to make the discs. However, if it were me, I would look for ways to integrate downloading from my website.
