I dont get the BPA

That really depends on how our personnel people rank these two players. They might rank Ellis above McFadden or they might rank them close. If they are close, then it probably makes sense to take a guy at the "need" position, but if they rank McFadden way above Ellis, you have to take McFadden. What good would it do to draft a player at a position of need if he doesn't end up being a good player? See, i.e Johnathan Sullivan.
When the scouts, coaches and FO types all stack the draft board, they should take need into account. So a better overall player at a position of less need might grade out close to a lesser player at a position of more need. When it's close, I think you have to go with best overall talent. The reason is that the need may be only for one year but that player is going to play for your team probably 4 or 5 years at least. You can't let short-term needs override your long-term goals for your team.