The tray in CD-based devices is a must, if you really care about things like mechanism quality, reading precision, jitter control, noise damping etc. There is a reason why home theater and profesional audio and video machines have trays. The clamping mechanism used in slot loaders cannot be guaranteed to keep the disc intact, and warping might occur in some extreme cases (prolonged use of the same CD, taken out and being put back in)
Slot loaders are also very sensitive to dust, and if dust particles get stuck in the right places, you'll scratch your CD every time you put it in. Read an audiophile magazine every once in a while, and see what kind of felings the people there have about this solution...
As for the mini-CD format being used... well, why not go for as much compatibility as possible? You ask if 8 cm CDs are ever needed - well, nowadays we rarely use the floppy, but most people like to have the drive present on their machines. And suppose you'll one day find a CD you desperately need to read - and oops, it's small, or Credit-card sized. What do you do?