Why exactly do we need these "trickle spawns", to make the waves last longer?
Because the game was designed in such a way where it's much more important to run 60 FPS with a gazillion particle effects, exquisite flora that would put to shame Rembrandt, backgrounds so marvelous you forget what reality is and a sun that would make you cry tears of joy till the rest of your life. Putting more than 20 enemies on the map at the same time was obviously WAY down the priority list. It's so obvious it should be that way, art is everything! We should have the outmost respect for Bioware's original vision... /sarcasm
More seriously, the person who designed the MP was an artist before being a programmer. Instead of making a game that worked for people that want to play it, they made a product that was for all intent and purpose "art"; Very beautiful, but ultimately useless. I see that every single day at my work, supervisors that have no idea what it takes to make it work, but oh boy they do love when it's beautiful and shiny.. it really doesn't matter to them if it can be used or not, if it can't they can always blame someone else...
Be thankful we now have BLuke, Blair and Billy to actually knock some sense into this game, it was a disaster from top to bottom.