As said before: I've been happily touring with the 2-speed, even in the Alps and know some people that have done the same. Each year a group of Brompton riders does the Berlin wall way on Bromptons - 165km on a single day. Some of them on 2-speeds and they are just average riders. Admittedly mostly flat terrain but I would not call this exactly commuting in flattish cities.... Each year in autum a group of Brompton riders climb the Brocken, a mountain in the German area of Harz. Some of them on two-speeds, some of them with an additional 39t granny gear. Not directly commuting in the flat either and no city around.

A useless discussion anyway as the OP did not say what he intends to do with his Brompton - I'd assume if he wanted to go touring he would have mentioned it.