I don't really do multi modal transport except for the odd occasion but the convenience of being able to take it in places, fold it up and put it in a supermarket trolley and simple front bag options appeal to me. Is this indulgent madness?
Those are two of the main the reasons I have mine; effortless luggage capabilities and not having to lock up. I've never done multi modal transport in the year I've owned it.
I also use the Brompton as my "wet weather bike" if weather is absolutely terrible as, unlike my other bikes, it has full mudguards (and brakes).
In addition to all of the above, the option to wear casual shoes/clothes is a huge convenience too.