There would be a lot of plusses to signing Owen- he's a free transfer, and a damn fine goalscorer. On the downside he is injury prone, and his wages would be steep for, essentially, a bit part player. Fergie could probably find someone younger, fitter and (even with a transfer fee) cheaper.
Newcastle definitely need to clear the decks, the question is how much it will cost them to do that, and whilst they probably do need players who will play for Newcastle, they also need fans with a slightly more realistic opinion of their club's place in this world. Be very interesting to see if St James keeps filling to the rafters when the likes of Blackpool or Plymouth come calling as well.