I have to agree with this. From my younger car-modding days, people were often effectively replacing a deliberate and specifically engineered design that in all likelihood made the best compromise between ducting/sourcing air, maximizing flow, and protecting the filter element for filter longevity and serviceability with a tube whose engineering design rigor was likely the equivalent of "we made it fit in your vehicle's engine bay" as well as fitting an exposed aftermarket filter that operates exactly as mrtn says here.
With just the K&N filter, yeah you might gain a little more flow in the short term (why do this in a cargo van), but it is at the expense of allowing more dust/particulate into your engine combustion chambers. K&N as a business has been a great success in marketing if nothing else.
As for the moisture and humidity issue with OEM air filters, I can't say I've ever had that issue in the Pacific NW. While the temperatures in the bay would tend to bake out any moisture and humidity in operation, perhaps air filter degradation could be an issue if the vehicle sits unused for long periods of time in a humid climate. Even so, your standard air filters are cheap to replace and easy to access to check for damage.