So you say, well, your DVD will be 'in the cloud'. Never mind that pushing that much data up most connections will take all month, how will he even get it onto his computer with no optical drive in the first place?
Well, you might say, all your new stuff will be on the cloud, so don't worry about it. But consider if he has a massive DVD collection. Heck, not even a massive one, just assume a hundred or s DVDs.... are you Seriously proposing that he repurchase all that media in 'cloud' format? At $30 a pop, that would easily justify the cost of going for a more expensive MBP rather than a crippled Air.
Oh you say, well if it is just a matter of money, you can just buy the external optical drive. But now whenever you take the laptop anywhere, you have peripherals to drag along with you, and that involves logistics. Again, he's better off with the one that is just built in.
Disclaimer: personally I think the Air is pretty neat. I just don't agree with parent's assertion that optical drives are unnecessary.