The reason for a switch is so you can disable it when you don't want the antenna to automatically deploy when you turn the ignition or the radio on, such as when the car is covered with a thick layer of ice. But you still have to remember to turn the switch off.

I've had 3 cars with power antennas. A 1973 Datsun 240Z whichhad the antenna broken off by vandals because it didn't automatically retract when I shut the car off, a 1983 Camaro that destroyed the antenna when it wanted to deploy when frozen, and my current 740 Wagon, on which the antenna appears to be stripped. I have a replacement mast, but have never gone to the trouble of repairing it. In my opinion, power antennas are more trouble than they're worth.
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1992 745, >500k km