A shroud has more to do than just increasing cooling. Air will always take the path of least resistance, and a shroud helps direct that air flow through the radiator. This of course keeps things cool, but also helps increasing the air speed through the radiator. This is important when it comes to removing dust and dirt, especially around the edges and corners where air flow is weak. As dust builds up, it acts as insulation and reduces the effectiveness of the cooling fins. Once dust starts building up, it's like a magnet and catches more and more dust. This is also why you flush a cooling system, to remove the crap inside that is causing the heat transfer to be reduced. You should also hose down your radiator once a year or so to help keep it clean.
I'm sure we've all seen those gold shows on TV, and they worry about too much water flowing through their sluice boxes, because fast moving water, like fast moving air, will suck out all the gold or dust on cooling fins.
The other thing a shroud does, is it puts equal pressure on the fan blades as they draw air. Remember, air takes the path of least resistance, so if the blade sucks air near ground level where it's open, but as it rotates to the top, it has less air to draw, the fan blades start to flex back and forth, and they can crack over time. I've even seen them break, and then you usually need a radiator along with a new fan.
It's also hard on your water pump bearing and I've seen the lack of a shroud cause them to start leaking prematurely. You don't want high and low pressures for a blade to pass through.
This is like a water pump or outboard motor prop that starts to cavitate. Nothing good comes from this, and while air is like 800 times less dense than water, eliminating it, if you can, goes a long way towards preventative maintenance and premature failures.
Also, this time of year you see people blocking their radiator with cardboard, which is fine, but cut a ROUND hole a few inches across, where the center of your fan blade is, so the air being drawn in, is in the center of the blade and the fan doesn't experience flexing.