I would have to agree with the your installers reccomendation.
Eliminating the transformer is eliminating another potential failure point.
By tapping into the voltage at the DI motor/pressure switch, you will have the humidifyer powering up only when the furnace does instead of drawing constant current and operating only when needed.
you eliminate another failure point, and save some electricity as well.
It should be very easy to do. Just replace the current single female end sta-kon connector thats hooked to the switch with a dual male and female end connector. Then attach a female end sta-con to the your new wire running to the humidifyer and hook it to the male end coupler at the P-switch.
The HUM should be what you are looking for. To verify check for voltage comming off that terminal. If there is voltage in heat mode and while the blower motor is running, that's your terminal.
The only disadvantage of having the humidifyer voltage supplied from the blower vs the inducer is that it will run for short periods of time when the furnace is off because the blower runs for a short period after the furnace shuts dowen to remove excess heat from the heat exchanger.
HVAC Technician
11 years experience in repair and replacement of residential and light commercial HVAC. CFC Certified