I personally think that lacking on optical drive is drawback. I have lots of software on cd/dvd roms. Downloadable software is great....but what's the first thing I do when I purchase and download a 2.5gb software product? I burn it to DVD to have a hard copy, and file it, rather than let it just eat up my hard drive space. Then I write on the dvd, labeling what software is on it. Can't do that with a thumb drive.
Also when you want to dump some software/media to disc to give to friend/family, blank dvd's are so cheap. I'm not going to have a stack of thumb drives on hand that I can just 'give away' to friends or family.
Also, the mini has been popular as a home theater/media server. Not being able to pop a DVD in to watch a movie kinda removes the mini from that role for a lot of people. It's bad enough that Apple refuses to put blu-rays's in.
Saying "well you can hook up the external dvd-rw drive"......I mean that kind of defeats the purpose doesn't it? Having this sleek small all in one computer, and then having this drive dangling off of it.
But, regardless, the market will ultimately decide the utility of internal optical drives.....