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The drain pipe behind the wall does not perform the same task as say, a drain pipe leading water away from a house tied to a downspout....
The purpose of the drain pipe is to evacuate excess water pressure......not necesarrily excess water. the pipe basically is acting like a sponge, sucking out the excess water and then ejecting it out the opening. The same result can be achieved by having weap holes in the front of the wall, but due the fact that in many cases, such as along driveways, it is not practical to have drains dumping out of the front of the wall onto the drive. Instead, you want it to dump somewhere else.
In all honesty, I have very few walls where I ever see water come out of the drain tile. It really is a worst case scenario, where a large amount of water builds up and needs to be removed.
What happens with a perforated pipe is the water isn't taking away, but more or less 'distributed evenly' across the back side of the wall.....in other words, it is likely that one spot behind the wall may have more water than others. The perforated pipe more or less distributes areas of high saturation to areas of lesser. In a drastic situation, when the entire area behind the wall becomes saturated, it then flows out of the pipe where it is daylighted.
More or less, the pipe is a pressure relief valve....not really a drain pipe that we more commonly think of.
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