I think that because the players must stand at least on, and normally behind the baseline in order to defend against the extreme deep ball (within 2' of the baseline) that the highly topspun balls hit a bit shorter are effective because the relative shortness gives the topspun ball more time to gain height.
In other words, if you hit a heavy topspin ball shorter, it will kick up higher before reaching the opponent who is behind the baseline, and be possibly more troubling to them than that same topspin ball hit very near the baseline which they would have no alternative than to short-hop.
With the heavy spin/poly string, it is more difficult to read where the ball is going to drop, and so one must still stay back in case the ball lands very deep. By the time it is recognized that the ball has landed short, it is too late to close in and attack it, as the ball will rapidly rise.
I'm your huckleberry...