I have an Acer Aspire One netbook that I have refitted with an SSD and carry on camping / geocaching / mapping trips where I want to be able to re-program my GPS on the fly yet don't want to haul something I care about (like my MBP) where it could get stolen or just die of bouncing around in my Jeep. It doesn't have an optical drive. I've used an optical drive one (1) time with it -- when I removed the original 120mb hard drive and re-installed Windows XP onto the 64GB SSD. I plugged it into the USB port, did what I needed to do, unplugged it, put it away. Someone wanted some photos I'd just shot and loaded onto the netbook to sort through and upload to the Internet (this was at a campground that had wifi - yes, they exist). I just loaded them on a USB stick, handed it over, he stuck the stick into his own laptop, sucked the files off of it, handed the stick back to me. That's how it's done in the real world -- I never carry blank DVDR's with me anyhow, but I always have a USB stick zipped into one of the compartments of my laptop case, you can get them so tiny that you can barely see them sticking out of the USB port now, which makes them *much* more convenient to carry -- and much faster, too.
In other words: For the 13.3" MacBook Pro, I would happily give up the optical drive in order to get a larger batter and faster graphics. The only time I've ever used it is to install operating systems -- Snow Leopard on the main partition, then Windows 7 via Boot Camp -- and a plug-in optical drive would work just as well there. I have a nice small plug-in drive made for laptops that is USB powered (just plug it into two USB ports) that works plenty well for that (I bought it for installing Windows on the netbook, but I've plugged it into the MBP and it works just as well). Right now I'm hauling all that weight around for no good purpose, I'd much rather have a couple more USB ports, better graphics, and mo battery to go with them, thank you very much!