If you do some test riding,you will find 20" wheels are fine for just about any purpose. A brompton does not have the same geometry as a 26" bike, but are common bikes. Sometimes people extrapulate that all folding bikes ride like a brompton.

this is not intended as anti brompton statement, as the bike furfills its intended role well.

Try a dahon speed pro, a birdie, a moulton a bike friday, a downtube FS ,a dahon jetstream,a dahon hammerhead. If you try any or all of these bikes you will be convinced that a bike does not need 26" wheels.

I am currently trying to get a friend who is considering taking advantage of the scrap car initiative to buy a new car to consider getting the bike to fit the car ,not getting a car to fit the bike.
I have suggested that she tried a birdie demonstrator out at the LBS. Her preconception means that she probably will not do this, yet I know the bike would be perfect for her intended use.

Its a slow process, but things will change, if you want to build a folder build a better 20" as none of them do everything well. Else try a 26" ,it may sell, but it will be less useful as a folding bike.

just my 2p.