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My opinion is it helps because there's not air flowing through the engine bay. While there is no coolant going through the radiator (or at a significantly smaller amount), there is still air flowing through the engine bay.
On my Focus, with no grille blocks, and being driven very easily, it has a hard time getting above 160F coolant temperature. I can , however, grab the hose on the other side of the thermostat and there is NO heat and the radiator is stone cold. In fact, I drove it for about a half hour very easily and took the rad cap off. I could stick my finger in and it was very cold.
With the grille block, it does warm up a bit quicker. However, I have to be careful, with the COMPLETE lower grille blocked and the upper grille, it will get a bit toasty on long highway runs. Next year, I am going to leave a 2x3 opening in the middle grille, block the upper and monitor temperature with my netbook instead of the on board "engineering mode".
On my focus, a good amount of heat comes OUT of the top grille ... which is really odd. I originally blocked the bottom grilles when I used to do a lot of short trips, but didn't notice a huge decrease in warm up time ... when I stopped, I noticed that all of the heat was coming out of the top. With the top one completely blocked off, it heats up much quicker in the winter!
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