This is taken from something I read awhile ago about the subject and I can't really word it better.
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 necessarily excess water. The pipe basically is acting like a sponge, sucking out the excess water and then ejecting it out the opening. What happens with a perforated pipe is the water isn't being taken 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 day lighted.
Also, perf pipe can crush under the pressure of the back fill. Ridgid perf pipe is better. We also lay down filter fabric in the bottom of the trench oe behind the wall and do a layer of washed rock down, then the pipe, then more rock and wrap the whole thing like a big burrito. This will keep everything free of silt. I have built multiple houses in Seattle on steep side slopes and this is what we did at the footings.
Hope this helps.