I both wholeheartedly think it is unnecessary and wholeheartedly like the fact I have one installed. As others have mentioned, the first time you dump out a shot glass of oily sludge after 1-2k miles of driving, it kinda makes you a believer. Yes, I understand the hose setup may change the nature of the system. But I do not think the length of hoses is responsible for the dramatic oil collection you see with a catch can. So the PCV has a oil separator built in? Well a lot of oil is getting past it and into my catch can. I didn't change the PCV valve. It's the same one...letting oil get past it and into my catch can. So even if the length of hose changes the moisture content, the oil content is still an issue.
I may be wrong, and I'm OK with it if I am wrong. Nobody is good/bad, dumb/genius, or any other dichotomy because of whether you choose to install a catch can or not.