That's something I also thought about. But in my opinion, stuff like personal photos and videos should alsways be saved on - at least one - external HDD. This HDD can also be used with Chrome OS, can't it? But you will have to upload them if you want to edit photo's I guess.
The ChromeBook won't be a replacement for every PC, but e.g. my parents are only using the PC to check their mails and use facebook. A ChromeBook would be perfect for them!
The lack of "real" client side software isn't such a big disadvantage, but there has to be some kind of "offline mode" where you can use app's like Google Drive.