Good observations Dan,
I spent a lot of time researching Darrieus turbines and even building a bunch of small versions. You're right about the area observations.
If we were to use your 10' diameter horizontal rotor as an example again, let's say there are 3 blades. Each one would be around 5 feet long.
So you would have to make or buy about 15 feet worth of blades.
If it was a Darrieus machine, to get the same swept area, each blade would need to be 15.7 feet long.
So for a 2 blade machine, you would need 31.4 feet of blades.
For a three blade machine, you would have to make or get 47 feet worth of blades, or over 3 times more blade length than with the horizontal rotor.
If you compare both turbine types to a wheel, the blades of a horizontal axis turbine are like the spokes of a wheel. Naturally enough with a Darrieus machine, the blades are like the wheel rim. Unless a guy greatly enjoys making blades it's hard to see the advantages...
However both types are fun to build!
Ted.