I have identified two situations where bicycles can be more useful than footbikes. This is not including long distances or luggage-carrying, both of which can be tackled with footbikes, just to a slightly lesser degree than bicycles, but situations where bicycles have a clear advantage: in rain and snow.
In the rain a footbiker must still strike the ground with every power stroke, including into puddles, and his legs also will be not protected by a rain cape, which on a bicycle is sufficient to keep one dry and ventilated. Capes are the choice of European bicycle police, who have been using them since the bicycle’s invention. (I’ve been using a rain cape on a bicycle and a footbike, and my legs stay dryer on the bicycle, where they don’t have to emerge from under the poncho for kicking.)
And when snow or ice is on the ground, studded tires on a bicycle will be sufficient to maintain grip and stability; those same studs on shoes, while good for kicking, will be unstable on a footbike’s deck. This is also where a kicksled might come in handy, but only in the land where footbikes were developed; in other snowy climes kicksleds are still relatively rare and still not as versatile as a bicycle. A bike with studded tires can still ride on pavement.
But what bicycle is closest to a footbike in simplicity? Probably one equipped with a coaster brake, and possibly an internally geared rear hub, also with a coaster brake. Next would be a single-speed freewheel bike with two rim brakes, and after that one with an internally geared hub with two rim brakes.
A fixed-wheel bicycle is even simpler than a coaster-brake bike, but some will need a shoe-retention system, the simplest of which would be toeclips and straps, so as to be compatible with most street shoes. (I’ve found that a fixed-wheel bike with a front rim brake to ride very nicely even without clips or straps – around here there aren’t many hills – but then I’ve put on a lot of fixed-gear miles over the years.)
A coaster-brake bicycle, on the other hand, can be ridden barefoot if the pedal is wide and flat enough. The fixed-wheel and coaster-brake bicycles ought to have a front rim brake in case of chain failure or emergency stops, but a casual bike in flat conditions can probably dispense with that, too.
I am buying a 700c coaster-brake rear wheel for my old singlespeed bicycle so it’s ready for the rare ride with less maintenance.
Update: here it is, although I’m afraid another upgrade is nigh – I just discovered retro-direct gearing.) I wear Waldies (the original “Crocs”) a lot, both with and without socks, and a coaster-brake bicycle will be a “jump on and go” ride without botherations like brake shoes or cables to maintain. That’s still only when I don’t choose to kick where I’m going, and that will be rare indeed.
Update II: The coaster-brake version of the Specialized Crossroads above was a lot of fun, but I found some blog entries about unclipped fixed-riding at Lovely Bicycle and decided to go fixed again, bolting on a wheel with a two-sided track/threaded hub. Mmmnh. When you ride a track, you won’t go back.