The rumour floating around a few months ago was that with the imminent departure of Robbie Keane, and Crouch + Pavlyuchenko not doing too well at the moment, we would be investing in a good, cheap striker; the name floated around was Edabayor from ManCity (would be a great buy, talented and would cost barely anything).
From reading Redknapp's comments on signing Beckham, the big reason they want him is because they want him to help out Aaron Lennon (our right winger). Lennon is one of the fastest players in the world and has skill but you rarely see him cross or score, etc, because he usually just uses his speed. Playing Beckham off the bench is not only a great move to boost our midfield but he would also provide a lot of experience and positivity to our young players, i.e. Bale, Lennon, etc.
He would be a great buy and wouldn't cost that much either, plus all the money he would rake in for the club.