Have people not been paying attention to the little bit of actual research done on the subject by Lindsey, Cross, et al.?
Poly strings slide easily against each other. According to research to date, there is an actual "slingshot" effect with the strings flexing and snapping back during the ball's dwell time on the stringbed, thus helping the player put more spin on the ball. While this happens with any string type, it is much amplified with poly (or any strings sliding easily against each other...hence the silicone spray thread here).
Yes, use of poly may affect spin production in other ways -- if the string is less elastic and less powerful, the player may feel more free to swing out hard on every shot.
But the "slingshot" effect appears to be the crucial difference. Try poly at 30 pounds in an open stringbed and you'll feel it.