I appreciate there are differing views. Briefly, the student is usually initially over-whelmed by the complexity of a cockpit in the real aeroplane and even the noise level of the engine which is very loud even with a head-set.
He won't have a clue on the radio selections let alone what is being said over the intercomm from ATC and his instructor. If it is bumpy, and there is the slightest hint he is feeling queasy, then further instruction in the real aircraft is a waste of learning time and he may as well return to base. He has just forked out anywhere from $2-300 bucks for the experience so far.
If the instructor's concern against a synthetic trainer to start with, is purely based on the fact the student might not look outside when he first flies the real thing, well surely it is merely a question of his instructor reminding him to scan outside as well as scanning his engine and flight instruments inside.
After all isn't that how you should fly, anyway?
With a fixed pitch prop we all know that a regular check of the desired RPM is needed and the student has to look inside the cockpit to see that as well as scanning his altimeter and heading and airspeed. With early synthetic trainer experience, this will enable the student to scan these instruments naturally. Same principle with radio procedures, transponder selections and physical touching and switching of radio knobs in the synthetic trainer before venturing in the real thing. At least the instructor can "freeze" a synthetic trainer to teach and discuss which of course cannot be done in flight.
I am sure most first time students who are generally keen and enthusiastic and don't have much money, will have already "flown" a Microsoft Trainer even though it might be a 737 or a C172. Would an instructor then throw up his hands in horror and tell an enthusastic student to never ever play with a microsoft trainer lest he forever be doomed to fly with his eyes inside the cockpit and run into another aeroplane? I jest of course but you get my drift...