The frame does flex. The factory rod will flex along with the frame while a solid will not. I have heard this plenty of times.
Now from what i have seen and putting about 200,000 rounds through a gen2 G17 with a solid rod, i never seen any adverse reason to not use one in one of my guns. Seen plenty of other match shooters week after week with solid rods and none of them had a problem with them.
It might be true and i might have worn out my gun faster shooting with one, but i put about 150,000 through the gun with factory rods and springs before i switched. Not a difference at all from what i could tell...