Sea Yoda, the short answer is that Rain-X contains several alcohols that damage acrylic ports.
For those further interested, here's the long answer:
Optical-grade acrylic, polymethylmethacrylate, is a polymer. Anything that breaks the polymeric backbone will result in cracking, or hazing (which results from microscopic cracks).
Materials that mildly solvate the backbone produce these delayed imperfections. Generally, all distilates from petroleum so perform. These would include gasoline, mineral spirits, diesel, Stoddard solvent, etc.
The backbone may be broken by removal of the acrylate active group by common bases, such as ammonia or sodium hydroxide. This may be also be accomplished somewhat less elegantly via "brute force" by common oxidizing acids, such as Nitric and Sulphuric.
Hydrocarbons whose structure forces a presentation of an electronegative group will easily solvate strand-to-strand interactions, causing immediately damage and flexibility. Some of these materials are 2-butanone (MEK), acetone, dichloroethane, etc. Those with groups of lesser gradient that are similarly "presented" will act by causing cracking and hazing, such as isopropyl alcohol, phenol, ethanol, t-butanol, etc.
Most "rain repellents" work because of silicone. Molecules containing multiple atoms of silicon, siloxanes, are placed into microscopic surface imperfections. Water then cannot cling to the silicon, and thus, the glass (or other substrate).
Rain-X uses several alcohols, ethyl alcohol and isopropyl, to act as a carrier for the siloxanes. This has the added bonus of cleaning the surface imperfections, allowing better grip by the siloxanes, and better durability of the applied material.
RVR places the siloxanes into an emulsion with water. This makes application more difficult, and less durable, than the Rain-X formulation, but will not damage acrylic.
I guess this a point for expensive glass ports.
Whew! Class adjourned.
All the best, James