http://bicycledriving.org/public-awareness/motorists-should-...
(tl;dr: It ain't your lane)
There are a ton of reasons for not being in bike lanes including snow, pot holes, glass, other debris, and the very real potential of getting doored.
Don't see why that isn't true for bikes.
But it really disrupts traffic when someone is trotting along at 15. Be that a horse, dogcart, or tractor. In a car-centric infrastructure, it's a disruption. Obviously the USA infrastructure, particularly in rural areas, is car-centric and thus non-cars will be disruptive. That's my point.
It all looks car-centric when you're driving a car. In rural areas, if you've even been in one for even a short amount of time, you learn that the roads are almost exclusively there for the commercial traffic. Iowa has a grid on a 1-mile spacing of gravel roads - not so you can race to grandma's house in your car, but so all arable land can be serviced by tractor and truck.
Let's say you're traveling 10 miles in either bike (15 mph, very fast cyclist) or car (55 mph). It will take roughly this many minutes to travel:
(* 60 (/ 10 15.0))
40.0
(* 60 (/ 10 55.0))
10.90909090909091
Note that I live in a rural area and bike around as much as I can. So I swing both ways here, and I still say that bikes disrupt road infrastructure which isn't designed to handle them.