AWD car needs a new tire; Replace one or all 4

Went to Firestone today because I have a slow leak in one of my tires. The car is a 2002 Subaru Impreza TS with factory spec tires 195/60/R15. Turns out the slow leak was due to a puncture in the sidewall of the
tire, so that tire is garbage. The tires are all about a year old and have decent meat on them, but the tech said you can't just replace one tire on an AWD car, you have to replace all 4 as a set, because the new one will be some sixteenths of an inch larger than all the rest.
Of course he'd say that :D
Now, I understand the concept and theory of this- if all wheels are the same circumference, they all rotate the same amount and internal drivetrain components (center diff or viscous clutch) don't have to take up the slack. But... I have a hard time believing that an amount measured in sixteenths of an inch is significant. Surely a few pounds of pressure off, or an unbalanced load (i.e., some fatass sitting in a seat opposite some 80 pound child) would make just as much difference?
I declined to make any purchase at this time. I know I'll need to replace that one tire, but I have another vehicle to drive for now. I would like to replace the one tire only. Replacing all 4 is a bit unnecessary, right?
Thanks
-J
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I immediately thought of a guy here who run mismatched tires on front and back axles of a quattro audi (torsen based one, not haldex crap based one obviously).
the drivetrain was making all sorts of noise to a point that snake oil salesmen offered to ditch the driveshaft essentially converting the car to fwd.
all the noise was gone once the guy had installed 4 matching tires audi requires.
the drivetrain on subaru would've been probably ruined by the same treatment but torsen seem to take this kind of abuse better, or so i would like to believe
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