The defrost setting is the last one that needs vacuum. As a matter of fact, it doesn't need any vacuum to go into defrost. Defrost is the default setting if vacuum is lost, so that you will be able to see out the windshield in case of freezing or condensation.
You can pretty much get by with the A/C blowing in the wrong place, but if you want to be able to see, you'd better have defrost available, so it is set as the 'default' position of the mode selection 'door' as a fail-safe.
If the TPS -throttle position sensor - were to fail, the computer might think that you had your foot on the gas pedal when you did not. It would then 'prime' the IAC to be ready to go into action when the speed sensor indicated you had come to a stop. It, the computer, also uses the IAC to compensate for the loads of the A/C compressor and power steering pump when the engine is at low rpms, in addition to its cold fast idle function when the engine is first started. If your idle sticks when you start from cold with a high idle, you may want to clean the IAC. The TPS can also have 'dropout' where the sweep is worn and the voltage goes to zero. That will fool the computer into kicking the IAC into gear when you are rolling down the highway. Complex, no?
tom