The perceived purpose of the catch can is to prevent the intake valves from carboning up and many believe that this is due to oil entering the intake system via the pcv and burning on the backside of the valves. While this may be true to an extent the main contributor is valve overlap. Valve overlap is the amount of time expressed in degrees that both the intake and exhaust valves are open. While this is happening some of the unburnt mixture is forced back into the intake and adheres itself onto the backside of the intake valve. This was never an issue until direct injection came into play because raw fuel was always flowing over the intake valves and cleaning them off. Once this buildup happens nothing short of walnut shell blasting will get rid of the buildup.
Many European engines utilize some sort of system to get rid of oil vapor in the air but in terms of carbon buildup on intake valves they were not very successful. It was not until Toyota (I think) developed the port and di system combination did this finally cease to become an issue. Since then mfgs have been adopting this system due to it's various advantages.
I have never seen any evidence that the presence of normal amounts of oil vapor from the pcv system in the intake air has any affect on performance. I'm not saying it isn't there I'm just saying I haven't seen it. Now on engines that are used for, or built for flat out racing this type of system is beneficial. These engines tend to run at high rpms for extended periods of time and if enough oil gets into the combustion chamber it can start to foul plugs.
I can not tell you how many vehicles over the years that have rolled into the shop with issues specifically related due to catch cans. Everything from check engine lights to excessive smoking. Granted most of those issues were due to improper installation but there have been some where everything was installed correctly and they still had issues. I also service A LOT of 3.5 EB's, many are fleet vehicles in excess of 200,000 miles, and have no issues with intake valve carboning. Would I expect to have an issue with a kit sold by Ford? No, but I also don't see it preventing a problem that isn't an issue. I don't see that this part has a CARB exempt number on it, which most people couldn't care less about, but it does add to the validity of this being effective in my opinion...I guess that's why they list it as a "dress up kit" YMMV