These are good points. Two smaller batteries may be ideal for weight-conscious backpackers. The Powercore II that I use is very heavy and I know many backpackers that would prefer not to carry that much. It's all about finding the balance between your intended use and the gear you carry (for example, 10,000 mAh of capacity is likely enough for most users, especially solitary backpackers, in which case it might even be better to use even smaller 5,000 mAh batteries for the redundancy you mention).
I do regularly leave the grid, but not always on foot, so that changes the calculation as well. I'm much more inclined to take a larger battery (or multiple large batteries) on a rafting trip, for instance, where gear weight isn't such an issue. Also, if I want to power a speaker all day (as on a multi-day rafting trip), more capacity is absolutely a necessity, especially if I power my comrades' gps watches, speakers, phones, etc. Everybody wants a charge when they see you have a solar panel