I had the last model of iBook with notoriously bad memory controller, and a MacBook that had sleep problems, and the MLB was the smallest replaceable part.
Still smaller than the part replaced when the display on my iPad developed a discoloration; walked out of there with a whole new unit.
Oh I'm not saying it isn't needed sometimes, it just seems like the code word for "no clue" or "nuke it from orbit, only way to be sure". It has just happened to me way too often to not be a little suspicious.
I would love to get the figures on the "whole unit iPad repair / sell old one has refurbished".
Still smaller than the part replaced when the display on my iPad developed a discoloration; walked out of there with a whole new unit.