Interesting... I've had a very different experience with these windshields, just replaced one not too long ago from a reputable shop.
From my experience and talking about this very issue in depth with a glass shop guy, the windshield weatherstripping system used on these trucks was one of best designs, also the simplest, and very easy to replace - guy said back in the day he used to do them in a bout 10 minutes because the system is so basic, and he misses the old stye because they worked so well (best design out of GM in his opinion) (although he doesn't see that many any more for obvious reasons).
He also said that if you have a good windshield weatherstrip in place already (see: OEM quality/rubber) and one that's been in the vehicle for a while, then it's a good idea to continue using that one. Getting a new one won't seal as well since the old one has formed to the windshield frame exceptionally well over time. Plus, aftermarket weatherstripping doesn't come close to OEM quality, and finding a NOS is next to impossible, and degradation of rubber may have set in over time.
I trust this guy, but could be wrong, you never know... But, in fact, I originally wanted to buy a complete replacement weatherstrip with new chrome locking strip from the shop, they showed me the new parts they had in stock (looked great), but the guy turned me down after explaining what I had in my truck was better than anything I could buy new today.
If you're leaking, make sure your frame and pinch weld has been thoroughly inspected for rust or damage, and if you're replacing a cracked windshield be sure to see how and where it cracked, that could give clues to a weakness or irregularity in the windshield frame, and subsequent installs will have problems if not addressed.