I live midtown, NYC. There are places where I simply cannot or do not want to go indoors with my Birdy or wife's Brommie. Trader Joe's, Metropolitan Hardware, Museums, etc. I have a 3.5 foot Fuggedaboutit Kryptonite chain attaching the frame to secure racks + a cable through the wheels and through the seat. The cable can be snipped if thief wants to bother with the evident tangle. But the chain must be attacked with a grinder (est. time is about 4 minutes.) Even in NYC, that's a little much for busy daylight environment.

I have a well-fitted backpack, but sometimes I transport the heavy chain in a bungied sack on my rear carrier. This is about fairly limited, just several mile, distances, not touring.

So don't jump to the conclusion that folders are unlockable and only an idiot would ever have a reasonable occasion to lock. I haven't had the bike that long, and if it's stolen this thread will be the first to learn of it along with my apology. Until then, use common sense, spend the $ for a chain, and enjoy.