Tom,
If you see conductivity between the center pin and the shield with no loaded 
antenna such as the Diamond 
connected to the mount, you have trouble. Such a reading would mean a short 
circuit, and thus your rig would go 
into shutdown mode when you transmit.

With most five eights wave antennas, you see a DC short because of the way the 
matching coil is 
connected between the antenna and the feedline.

You won't see this short with a simple quarter wave that contains no matching 
coil.

I have a similar, but smaller window mount from MFJ. It is intended for use 
with an HT and rubber 
duck just like the info you posted says. I get good results when I use it with 
a quarter wave antenna. My antenna 
terminates in a pl259, so I adapt it to a BNC, and I'm in business.

I added two radials to this mount by slipping two pieces of 26 wire under the 
screws that hold the 
mount together. These wires remain inside the car, and I keep them in place 
near the bottom of the back passenger 
side window with small suction cups. The setup resembles the printed letter Y. 
The addition of the 
radial wires appears to improve the performance of this arrangement.

Of course, a magnetic mount in the center of the roof will beat it, but this 
keeps the wife happy.

I think my mount is rated for 20 watts, but I have only used it with an HT.

Your Commet mount sounds like a good setup.