Yes I realize that. It seems strange to try to save weight with plastic components when the frame would be the place to do it. I worry about the durability of the plastic and the design choices that lead to it. I also see that Bromptons still have cup and cone bearings and the fiddling required to get the rear wheel off cancels out the elegance of the fold IMHO. I'm also looking at the Tern verge x10 as I like the low weight and the reviews say it rides very well. The fold isn't as nice as the Brompton, but it's probably good enough and I can mess around with the gearing as much as I want and change out wheels easily.
The Brompton frame is too heavy IMO, really. I don't know much about steel tubing but from what I gather Brompton uses straight gauge high-tensile steel. . . someone can fill me in on whether this is the right choice in design terms. I guess there's only scope for butted tubing on the rear frame, but surely some form of chromoly steel would result in a much lighter bike. But high-ten is cheap and Bromptons sell for big figures so I guess that's nice for them, and not for me. As for the use of plastic I tend to think it works well. The seat post q/r lever, for example, feels nice in plastic and it's preferable to e.g. the lower-end Dahons that have hard-edged alloy q/r levers. Not that it makes much difference. Brompton use plastic for the seat post shim, Dahon aluminum. It's arguable which is better, but Dahon's version innevitably scratches the seat post, abrades it over time, so I like plastic there too. The individual parts of the chain tensioner are available as spares so if you bust one you probably don't even have to buy the whole thing again. . . Brompton are good like that.
Cup & cone hubs are fine with me though Brompton do have a super-light front hub that I'm sure has sealed bearings, and is rather over-priced compared to the nice Dahon/Tern equivalent.
If you're riding longer distances and like to pull on the handlebars then I'd say the Tern would ride far better. Bromptons are flexy and have an odd gearing system, fine for shorter distances. If you like comfort then you may find a Tern harsh without some form of rear-end suspension, a sprung saddle etc.. I like Brompton's rear suspension.