I loathe Andrew. But I can't blame the queen for having him there or him for attending. It was his father, after all.
I think he came as her attendant to avoid all the weirdness re precedence. They either fudged it, or purposefully demoted, his daughters - who came in with Anne rather than after Sophie and Edward. But in the normal course of things, Anne (and possibly her children) would have arrived first, then Edward and his family, then Andrew and his family (or, quite often, his daughters, THEN Andrew). Theoretically it would then be william and kate then charles and camilla then the queen.
This allowed them to avoid all of that. None of his siblings - namely Ann or Edward - had to allow him to be considered more important because he wasn't there as Prince Andrew, Duke of York but rather, Andrew, son of the very old, somewhat frail queen ie unofficial. Or at least, that was their plan I think.
Of course, it all fails miserably because all it does is look like the queen is giving two fingers to the rest of the world because none of them can see that. Or possibly, the rest see it but can't do anything because mummy isn't interested in what the world thinks, she wants to protect her boy. So.... [shrug].