[Perfectlap]

that's a good question I never thought about whether the Plexus keeps the plastic from getting brittle. I lucked out on Ebay once and got a case of small plexus cans for $20 so I use it allot and give so to friends.
Judging by how an untreated miata or boxster window looks like its about to crack after the next lowering I'd say Plexus does do some good in keeping it flexible.
I also think that people will smear whatever is handy across the plastic to try and clean it like ajax or something they dug up from house cleaning bin. There's a product for everything.
Spray the plexus on the towel away from the car to keep it from getting on the canvas. Wash it off right away if you do get some on.

p.s.
This weekend I washed the top with Raggtopp and my OXO feathertip brush and I ran out of Raggtopp protectant half way through the re-application. It rained yesterday and you could clearly see where the water was penetrating and where it was beading.