jrod stated:
Possibly, but quite frankly, poly or co-poly's are not a good idea if you have any issues with TE or GE. In addition, they are not very econmical since they all die rather quickly and you end up either replacing them often or playing with dead strings which is not exactly a good idea. Finally, many club players don't really have the game to benefit from poly in the same way pro's benefit....you really have to take a big cut at the ball to get the most out of poly or else you're not really benefiting from the string's unique properties.
Spot on. I would say, however, that if you are a consistent player and big hitter, poly strings are worth the ride, at least once. There is nothing like the sensation of going all out and feeling that you just can't miss. But, as jrod points out, poly dies so fast that it becomes cost prohibitive to use it all the time. Think about how many times pros change racquets during a match; sometimes strings break, sometimes they want a different tension, but far more frequently, they need a fresh set of poly.
With the harsher, or stiffer polys, I've found that they (or I) need a break-in period of about an hour. They can be fairly board-like at first, but then overall playability improves markedly. That can be quite frustrating with a string life measured in hours, and is even more constraining economically. I don't prestretch poly when stringing; maybe that would alleviate or eliminate the need for a break-in period.
To more directly respond to the OP, I think I too, intuitively consider putting poly in a flexible frame as being at cross purposes. Yet, my own experience with poly --- which has generally been good --- is in stiff frames at high tension. Even experimenting with poly at low tension in stiff frames has not been bad. However, when I pick up a flexible frame --- which I do from time to time --- I am in search of that smooth, buttery feeling that I would usually find with a softer string.
If you decide to try poly in a PK, tell us about the experience. It would be interesting to see whether conventional wisdom prevails. I would caution, however, if you do decide to try poly, that it can be difficult to tell precisely when the string begins to die. I will generally start to feel it in the arm (discomfort) before I notice a deterioration in playability. Don't leave it in too long.
Cheers!