My attitude is that the rain gear is just part of my bike, like fenders, that make it useful for riding in all weather. My last set that wore out, simply became porous over time. I got a couple more years out of each piece using a waterproofing spray from REI. Now the rubbery inner layer of the rain pants has worn through in the crotch area. I commute in my street clothes, and don't sweat a lot, so a rain jacket and pants works pretty well for me. I also use the same gear for hiking, or if I have to be working outdoors for some reason.

I suspect that the materials undergo some sort of chemical deterioration, so they would last the same amount of time no matter what.

One suggestion: If you regularly carry rain gear, put it in some sort of bag to keep sunlight off it. Most synthetic materials deteriorate under UV exposure, including directly from the UV, and from the ozone generated by UV.