You can always use portable drives, cloud, or a NAS to store photos. In either case you need a backup, storing everything on one laptop is a bit limiting.
I do use portable drives, Dropbox, and Amazon Glacier. I have four copies of my photos. They are, by leaps and bounds, my most irreplaceable data. I want every single one of them on my main machine, which makes automating backups to the external drives and Glacier infinitely easier. It is a dealbreaker for me, and I don't find $400 an acceptable price to pay to get past said dealbreaker. Well, realistically, $800, seeing as my personal Dropbox is at 850 GB, it would be silly of me to buy an un-upgradeable drive that would be teetering on storage space issues from the jump. Apple thinks it is reasonable to pay $199 less than it would cost for an entire second MacBook Air to upgrade drive space to 2TB.