The big problems with using an external disk with a portable computer are (1) the hassle of plugging it in and unplugging it and (2) *remembering* to plug it in for periodic backups.
Between point (1) and (2), and the passage of time, 90% of people (my humble estimate) are going to let it slide more and more, until backups are performed infrequently, just whenever they happen to think about it.
(If you use your portable more like a desktop, where the external drive can stay plugged in, then this doesn't apply so much. However, that seems like unlikely usage with the easy portability the MacBook Air!)
When backups are inconvenient, backup frequency becomes a trade-off between safeguarding the data and minimizing the hassle. Will you use the MacBook Air for your photo repository? How often might you add photos? How many are you willing to lose if the MBA's drive fails?
The greatest thing about the Time Capsule is that your portable only needs to be within range and turned on, and all new or changed files will be backed up every hour (or when awakened from sleep). So, quite soon after you load new pictures into iPhoto, you have a copy of them safe on the TC. You probably won't even notice it happening.
Some folks here won't touch a TC, but I've got one (a 1st-gen) and I love it.
I've replaced the original 500 GB drive with a 2 TB green drive, and it was very easy, even though they aren't designed to be user-upgradeable. The only "hard" thing was removing the large rubber foot pad to get at the screws on the bottom (hint: heat from a hairdryer and steady pulling). Then it's just a few screws to replacing the drive.