It's not only a means to reduce weight, but also a polymer frame drastically reduces the costs of manufacturing a gun. An all metal gun is more expensive not just because the material itself is more expensive but the machining/finishing process and all the overhead that comes with it adds to the final product price.
with plastic frame guns they only balance well with a full magazine. as the magazine empties they get really nose heavy lose balance and get more difficult to point quickly. all metal guns even aluminum frames lose very little in balance as the mag runs dry.
in terms of handguns, the polymer vs steel is a debate that will never go away, as both platforms have their pros and cons, and both have been proven as durable and reliable (what's more reliable, a glock or a 1911?), so in the end, it comes down to personal preference.
now when it comes to the polymer long guns like the AR-15 lowers that are out now, it's still a crap shoot. The original design of the AR was to be made of heat treated aluminum, and there are arguments that because it was designed to be built from aluminum, and that same design is now being made from the polymer material with different strength properties, that a Polymer AR is inferior to an aluminum one (much like if someone took the 1911 design and simply started molding the frames out of polymer)...these individuals do support the idea that a rifle receiver that was designed from day one to be built from polymer can be just as effective and reliable as a glock which was designed from day one to be polymer.
That said, I've seen the Youtube videos of torture tests, guys taking a polymer AR and doing stuff to it that I would NEVER do with my own weapons, and they never broke it, which would indicate that for the casual hobbyist shooter, a polymer AR will be just fine. However, I wouldn't take one into combat where there's always the possibility that your weapon ends up being used as a baseball bat, something that our soldiers in combat may face, but it's not something that you'll ever face when plinking out in your backyard.
For a CCW I prefer weight reduction. Now that doesn't exclude metal frame, there are plenty of alloy framed handguns out there that give you the rigidity of a metal with a much "better" weight for carrying. Personally I think you'd be hard pressed to find any facts or statistics that prove a steel frame will last so much longer than a polymer. Glock has been producing their plastic bricks for 30 years. Yes steel has been used in guns longer so it's easy to say you've got a 100 year old gun that still fires. Poly doesn't rust and takes abuse very well and that's all I need to find value in it.
For Home defense and plinking I want something large and heavy to absorb recoil and recover quickly.