Obviously every solution is always a compromise and different compromises are best suited for different people and their typical situations. For me with every bike the decision is what gets bolted solid to the bike, that I can leave it behind in the street, and what I need to take off. If I needed to take off a motor and battery for every errand I would make, it would become a sentence. However some people as a rule always take their folder inside. I don't do it and I would hate to have to do it or to take a significant portion thereof. Again all kinds of considerations come in for different people including obviously financial. Certainly a benefit of a movable system is that when it does not work on one system you can take it to another. Yet for Brompton to prosper as a brand they utterly need to test and perfect the system so that it works well so you can take a whole layer of considerations off from there.
I guess it depends on where you live. I would never leave an electric battery mounted on a bike here, and any wheel with a motor needs to be u-locked. And even then, you can come back and find it gone. I moved on from regular bikes to a folder because I got tired of wondering if my bike, or which parts of my bike, would be there when I got back. I never leave my Bike Friday locked anywhere, but I do lock up my Dahon Mu Uno. I bought the Dahon used, it's got stickers on it, it's a single speed...less desirable. But if I lock my Dahon up, I want a system I can remove and take with me if I motorize it. It takes less than 10 seconds to remove the 1Motor, connect the two parts, and it has a carry handle built in. I can fit it inside the Patagonia Atom sling bag I ride with, along with my tools. I think a lot of people would prefer to lock up their "beater" bikes and be able to bring the motor inside with them for safety. If I come back and the Dahon is gone, I'm out a couple hundred bucks. A dedicated electric bike is a theft target, and u locks are super easy to remove. I know, because I had to cut one off my son's bike when he lost the key - angle grinder took 15 seconds for a kryptonite u lock. Like cutting butter. You'd better have great insurance if you lock an nice e-bike up in any major city. Either you bring that Brompton inside with you everywhere you go or you take that battery bag off and have to carry it; no different than the 1M except 1M is lighter.