I base it on my personal experience. I used to have a 3lb subnotebook with an external optical drive, and I regretted it. Not only was it cumbersome to use the attachment, but there were several times when I was travelling without it (didn't think I'd need it) and ended up wishing I had it. One time was at a conference when they had given out software demos that I couldn't install; another time was in Japan where I had bought music CDs (unavailable in the US) that I wanted to listen to right away, but couldn't.

After a bunch of those experiences, I made sure my next subnotebook had an optical drive built in. Thanks to technological progress, it was even lighter (2.8lb) and smaller than my previous one, so I don't feel as if I gave up anything for the built-in drive.

Currently, I travel about twice per month, but I only use the optical drive on 2 or 3 trips per year. The problem is, I don't always know in advance which trips those will be! So I would either have to take it on every trip (and carry more size & weight than if it were built in to begin with) or take a chance and guess whether I'll need it on any given trip. A built-in optical drive adds slightly more bulk & weight 100% of the time, but I don't have to play guessing games about whether I'll need it or not.

I certainly can't say the same will be true for you or anybody else, but having used subnotebooks with and without built-in optical drives, I simply won't consider a subnotebook that lacks one.