Nobody is suggesting that the outcome of every case should follow what prosecutors think. What we are instead now discussing is the ridiculous notion that nobody should even care what they think. Well, pretty Aaron Swartz cares a whole lot about what they think, despite disagreeing strongly with all of it.
The subtext of this argument is obviously, "prosecutors are self-interested, and what they claim to think has nothing to do with what they really think, which is 'I am going to pursue whichever cases present the best opportunity for career advancement'." This, too, is a dumb way to think, because however much prosecutors may or may not think that way, judges and juries do not think that way, nor do they believe prosecutors are entirely animated by career concerns.
The subtext of this argument is obviously, "prosecutors are self-interested, and what they claim to think has nothing to do with what they really think, which is 'I am going to pursue whichever cases present the best opportunity for career advancement'." This, too, is a dumb way to think, because however much prosecutors may or may not think that way, judges and juries do not think that way, nor do they believe prosecutors are entirely animated by career concerns.