Old engines needed to warm up because oils were thicker back then. Older engines had thicker oils due to larger tolerances used in the manufacturing of the engine. Todays computer controlled machining allows for the mass production of tighter tolerances and the use of thinner and synthetic oils which, depending on the season, really don't require a 'warming up'. I believe that manufacturers recommend a warm up period so the catalytic converter has time to heat up and all of their emission specs are met while the car is in operation. It is also prudent to warm up an engine in colder climates because an oils ability to flow is reduced when the mercury drops. Here in Michigan I let my Explorer run for a while in the winter, I use Mobil1 5W-30 and still give it time to heat up. In the summer, I just start the cars and go, I have a 2000 Mustang GT, also a Mobil1 5W-30 engine, and when it is 90 out, ain't no need to warm up (or time!!). When my Cal Spec is done and I'm running 10W-40, even in the summer I will let it run for a few minutes.
Carburators also like a warm engine so they aren't running on the fast idle cam. After they warm up a bit they run on the regular choke operation, my caburator knowledge is a little rusty so if there are any carb experts out there please correct me on this last bit.