I see them as having different roles in the booth, trying to bring different perspectives.
Boone brings the game from the player's perspective. As much as I detest AB for that HR, I think he does a respectable job of filling that role. But keep in mind that he's a Yankee first and has the same bias against the Sox that we have against him.
Mendoza is supposed to bring the perspective of the "knowledgeable fan", a/k/a someone who's never played the game but has read a lot about it and tries to keep up to date on what's happening. I'll give to her that she's up to date but what she says often comes across as exactly that - up to date with what she's read but with no real substance.