Composite bodied mechanic's ratchet:
http://www.harborfreight.com/12-in-drive-composite-ratchet-62618.html
Mechanics tools aren't always all metal, in fact, it costs more and only makes them heavier. It's tradition that keeps an all metal ratchet alive, I have some composite ratchets and they work just fine. Only the gear teeth and a small rod down the middle are metal. All the rest of the shape was done simply because the maker chose a particular fabricating process.
Injection molding of rods is extremely cheap. Pennies per part and the cost is in the mold. Same for receivers, the cost for a lower is pennies. Add a few metal bits to connect to handle the slide recoil and plastic lowers do just fine - they take the punishment of .45 ACP all day long. So do the rods.
If anything it's exactly why polymer framed guns do handle the stress - they are flexible. Like a road tire they give on impact, absorb the force, and rebound back to shape. Unlike aluminum or some cast frames which are battered after a few thousand rounds and crack. Read the thread on rental gun use on arfcom and after 200,000 the AK's start cracking at the rear of the receiver. They weld them up but they are eventually junk.
I'd rather take a long trip on some polymer tread tires than solid steel ones. Let's not get too addicted to tradition or insist that only one material is suitable for use. A flexible guide rod does the job just fine. All it does is keep a flexible spring from collapsing off center which could entertwine the coils and cause a failure.
I've read an analysis of aftermarket parts added to guns, especially the 1911, and it pointed out that changing to a different rod than factory changes the dynamic recoiling of the slide. Whether that adds to or takes away from the operation within it's envelope of functional reliability can't be known unless the owner tests it before and after. No different than arbitrarily changing some engine part on the daily driver - it's speculative at best to say it's an "upgrade" without testing and measurement with data gathering and numbers. Plenty of posters report that adding a steel rod didn't make it "better." There's no empirical analysis to prove it. Just touchy feely.
We'd all be better off in a lot of cases leaving well enough alone. Hot Rod your carry gun with parts that have no demonstrated and tested ability to improve reliability? I've never seen the makers of steel rods post a 100,000 round side by side test against the factory plastic rods to prove it.
They never will. It's all based on emotional hype - the extremely conservative shooting community is no stranger to it, it's what actually drives most of their decisions.