Optical drives take up lots of space, use a lot of electricity, and have moving parts that can break easily in a portable device like a laptop. A computer like the Macbook Air would simply be impossible with an optical drive.

I'm willing to make that trade, especially given how uncommon DVD's were even by the time the MBA launched. (Although I'll note that my desktop has a BluRay drive and I wouldn't have it any other way.)

What benefit am I getting in exchange for the loss of a headphone jack? An extra couple millimeters of screen real estate?