The thing I'd say about Bromptons is that they give you options. I've got across Paris with a dismantled and bagged road bike, and with a Brompton. The Brommie won hands down - 30 seconds to unfold it, ride away. Ferries are much of a muchness, trains the Brommie is easier, and Eurostar it's a complete no brainer. Busses will take a Brommie but rarely a full-size bike.
For touring - yes, I usually find a Brommie is a bit slower than my other bikes. But does that matter? If I'm riding from Forcalquier to Apt, that's 64k, and that figure doesn't change if I go faster. My touring day is, more than likely, governed by where hotels are, or where I need to be to do something that only happens every so often, like get a bus to Sault.