Anne drives the plot. Agree she's not particularly funny (which would be better), but she leads to things happening and conflict between others.
In series 1 & 2, April on the other hand is completely superfluous to every plot and never has the best lines in the sketch. I can't see that any editor/producer would read that script and not put the red pen through her character. When you read the background the show runners say they loved Aubrey Plaza as an actress so decided she had to be in the show somehow. It's like that rule about not leaving in your favourite gag if it doesn't advance the plot or define vital characters. Again, the red pen comes out (I bet they'd have edited her out of the script if someone else gave it to her).
Aubrey Plaza does a great job on the character, so good to hear they have made her character more central. That's what they should have done in the first place.
As I say, I love the show, I just found it a shame that time was wasted vs the best characters who made me laugh and were in some way important to the episodes.