Before the advent of low-friction stringbeds (like poly), all stringbeds were pretty much "locked" stringbeds.
When the stringbed is locked, a flexible frame is more spin-friendly than a stiff frame, if all other factors are equal. The reason is that the flexible frame deflects more during the ball impact, leading to a longer dwell time, with less normal deflection of the stringbed. The lower amount of normal stringbed deflection allows the stringbed to stretch downward more (assuming a topspin stroke). The increased downward stretch of the stringbed translates into more topspin for a given swingpath.
But with poly strings (and other low-friction stringbeds in general), you no longer need a flexible racquet or an open pattern to generate a lot of spin. And stiffer frames have advantages over flexible frames (better directional accuracy, and more evenly distributed power potential across face of the stringbed).
I have a personal preference for ESP in dense-pattern stiffer frames (I consider my Blade to behave like a stiff frame). This gives nice spin but with low launch angle that is more independent of the incoming spin. The softer the stringbed (and more deeply the ball pockets or dents the stringbed), the higher the launch angle, and the more the launch angle depends on the spin of your opponent's ball. The stiffer the stringbed, the better the control of the launch angle. The ESP method allows you to maintain the control benefits of a stiffer stringbed (at higher average tension that would ordinarily "lock" if strung without tension differential), while also getting the extra spin potential and arm-friendliness of a low-friction stringbed.