Like you said, carbon build-up is pretty much an eventual thing all car owners will have to deal with if you keep the cars long enough or drive them a certain way for a long enough time.
The oil separator delays the carbon build-up cleaning by catching most of the blow-by. An aftermarket catch can attempts to catch the blow-by before it makes it to the oil separator. If anything, theoretically an aftermarket catch can will extend the lifecycle of the OEM oil separator just by taking on some of the "load". That in itself is worth the $25-$100 and 20 mins install time to me.
In case there's any confusion, blow-by is produced when the engine is not at operating temps. More blow-by is produced in climates with high humidity. This means that if your commutes are short enough that the engine doesn't reach operating temps, it will not be hot enough to burn off the excess blow-by. This is why aggressive driving helps, it gets the engine to operating temps faster.
Here are pics of the blow-by caught by an aftermarket oil catch can on Silly Wabbit's CX-9. This was 2k miles after the installation.
Chris_Top_Her also has an oil catch can installed on his 2.5 NA and he has mentioned before that he catches "a lot of blow-by".
The only difference between an oil separator and a catch can is that the catch can needs to be manually emptied. If one "filter" works well, and a second "filter" can be added without any impact to "flow", then the second filter helps much more than it hurts. Thus the aftermarket catch can is an improvement to the existing system.
