I use a product from Meguiar's called Natural Shine Protectant, but there are a range of products. I did use ArmorAll for years but this product seems to work better.
Years ago (many now I guess) plastics and synthetic rubbers and elastomers suffered over time when the plasticizers would break down (mostly from UV attack) and result in parts cracking, chalking or becoming brittle.
Keeping them protected with a good product helps, but the quality of the polymers used these days is generally much better. Having said this, it is very easy to see products (including motorcycle parts) that are still manufactured with lower cost and lower quality polymers that will still suffer degrading physical characteristics in surprisingly short periods of time.
Glass filled nylon and acetal parts are often the most conspicuous particularly where they are being used to replace what may have traditionally been a fabricated steel part or a cast alloy part, and BMW have had their experiences here too. I remember a friend purchased a brand new K75 and the main structural bracket holding the front assembly of gauges and lights etc. was an injection moulded part that had a nasty habit of breaking in service. Heat, UV, vibration and time conspire against parts that are not well designed for plastic.
Away from the more engineering components, I use the Protectant on my seat to maintain a soft and flexible material. Remember the "old days" when bike seats made from PVC would split and then the foam inside would soak up the first shower of rain and from that day on you would get a wet butt every time you sat on your bike. Made worse still by trying to apply duck tape which would at best provide a temporary solution, but ultimately it would squirm and then the residue of the adhesive would transfer onto your butt too.
So, although I tend not to keep my bikes as long these day, I still take care of the plastic and synthetic surfaces for the next owner to enjoy. And it makes the bike look like new.