Indeed, multiple fallbacks are preferable to one. And if somebody wants to have a mass-storage external backup drive and also maintain a mass-storage online mirror of their files... more power to em. I'm on board with that idea, supraman.
However, in my experience, it's difficult enough to get people to back up their files, at all... much less convincing them to follow-through on maintaining redundancy. The ordinary person usually stops at a single backup... for better or worse. And if only one is going to be chosen, I would choose an external drive... for convenience, flexibility, and speed.
Granted, online storage does offer certain benefits not offered by self-storage... including, as you mentioned, protection against disasters that could destroy your hard drive. I just think that very few people are likely to reliably and consistently go through the work of maintaining both an HDD backup and online backup.
If my house burnt down, my photo library would be missed... quite honestly, though, it would be the last thing on my mind when confronted with misfortune on that level.
So, I do agree... external hard drives paried with online redundancy is a great idea. Whether or not an individual will realistically maintain such a backup system is the real question, though. I foresee many people being unwilling to... and instead choosing one or the other, even if that's not the ideal way to go.
My method: redundancy on two external drives. If my house burns down... yeah, I'm screwed. Moreso cuz I'll have no place to live... much less so because my photo library will be gone.
Admittedly, for individuals that make considerable income from their photos, there may be more reason to protect their photo library against any and all possiblitity of loss. In which case, supraman's suggestion is the only way to go.