Having studied materials at university, I would consider engineering polymers to generally be superior materials for lens barrel and camera construction to metals for most uses - and the 100 L macro certainly doesn't feel insubstantial to me. The only L lens I've owned that does feel lighter than I would expect, but is not flimsy, is the 70-200 f4 L IS, and the only L lens I've had that's fallen apart is my ancient and metal-barrelled 20-35 f2.8 L, and that only after it was dropped - and it used to belong to a press photographer in its prime, anyway, so it's had a hard life.
LOL, totally agree with you. Same reason why soldiers perfer using MAGPUL pmags over the traditional 'metal' mags... THEY LAST LONG ENOUGH TO NOT ONLY CARRY YOU THROUGH ONE FIGHT BUT MANY MORE THEREAFTER.
However, the only issue with polymers is getting the tight tolerances and consistentcy that metals can offer. This is one reason why Apple sticks with Aluminium