The only problem with BPA is that you could end up like the Lions taking 3 wr's in 4-5 years at the top of the draft. Yeah, they ended up with C.Johnson, but they also got stuck with a couple of duds. (though the one they traded to dallas for a couple 1st rounders worked out well in the end.)
BPA is subjective. A player that can help fill a deficency will always have a better grade than someone who doesn't. Say Player WR is graded at 88(example) and Player DE/LT is graded at 82 or so. Player DE/LT will be drafted because it's not that big of a difference and the team could use a DE/LT more. It's like a free 5-10 points.