Murphy has been stranded in Avery Johnson's dog house for almost the entire season. Murphy is averaging a career low 3.6 points and 4.2 rebounds in just 18 games. Last year he averaged 14 points and 10 boards a game. He is a career 39 percent 3 point shooter PER (player efficiency rating) was an 18 last year, so overall he is a good basketball player especially if you're not asking him to be one of your 3 best players.
Murphy has been at the center of the Carmelo Anthony trade talks since he arrived in Jersey, but ever since Russian Mark Cuban walked away from those talks (for now), Murphy's name has been floated as a guy who could be bought out after the trade deadline (though right now GM Billy King says that's unlikely). ESPN's Weekend Dime listed Dallas, Orlando and San Antonio as possible Murphy landing spots. I see no reason why the Spurs shouldn't make play for Murphy. He is essentially a more talented version of Matt Bonner. That and you keep him away from every other contender who will make a play for him (Like the Lakers need a back up center who can hit 3's).
The Spurs don't need Troy Murphy, but Boston is a good example of why having an overload of bigs isn't a bad thing. You can always use more rebounders. Or in the Spurs' case you can always use more rebounding and 3 point shooters. Troy Murphy does both of those really well.