Honest Q: where though? If storing outside on a bike rack isn't an option, where can the folding bikes be reasonably stored while out and about? If I go to the store, for example, I see no option but to lug the bike around with me the entire time. At work, sure, I could stash it in the office during the work day, but outside of that I'm not sure storage really works.
I would say home/the office is already pretty big. You're right, you probably won't store your bike in the shop, but if you're shopping with a bike, you probably won't stay there for a long time anyway so the risk is quite lower. For what it's worth, I've seen people put them in some corner at the gym for instance, so it's sometimes possible.
I'm not saying it's perfect, but I certainly see the appeal for commuters, especially as a last mile option. You usually can't go with a full bike in public transports during rush hour anyway, so the usual alternative is to have one bike at each end, where they are outside all the time.
At my local store(s) there's plenty of parking space, but no bike rack or a railing to lock a bike. At the gym, there's a bike rack right in front but no parking space. For my commutes accessible by bike, a folding bike offers no current advantages. The option of a bike that's easier to take on the subway is certainly appealing though.
