Oh, your argument was that water's poor heat conduction was a failing? It's not.

Imagine if the distributed fluid was an excellent heat conductor. The heat that entered the fluid would leave it immediately, and by the time that it reached the radiator, it would have no heat left. That would be phenomenally inefficient.

The reason that radiators are shaped the way they are is so that the surface area of the contained water is as large as possible, so as to speed up the heat transference. On the other hand, cylindrical tubing gives the water as low a surface area as possible.

Yeah, copper was probably a bad choice for this, seeing as how it is an excellent heat conductor. Based solely on the heat properties, they probably should have used the same cast iron they use for the radiators, but that's brittle, heavy, and likely to corrode. Iron is actually a relatively poor heat conductor, which means it retains a lot of heat and releases it slowly, which makes it ideal for things like radiators and skillets. There's no reason that the copper pipes feeding your radiators can't be insulated, though.