> Couldn't you simply use pure silicone in spray form (similar to WD40) to coat your ripstop more evenly than a brushed application?
Well, the problem with that is that they are two entirely different silicone polymers.
The lubricant is in liquid form, and remains that way, so is likely to leave a silicone oil stain on anything it touches.
The silicone sealant is a different silicone polymer, probably a much longer, or chained molecule, that forms a 'dry' gel when it's set.
Silicon has a similar chemistry to carbon, so forms many complex hydro-silicon (aka silicone) molecules, rather like the hydrocarbon 'organic' chemistry. It's nowhere near as diverse in its range of chemistry as carbon, though.