Understand your point and the arguments of others who are lobbying for Farmer. They make perfect sense. Actually, I don't think the Spurs can go wrong with either. I wish they could keep them both.
My concern with Farmer is once Ginobili comes back and with the addition of Mason, who will obviously play ahead of him, how much PT would Farmer realistically get? Would he rendered irrevelant? Since he's a D-League veteran, he's probably not going to develop much more there. And it would be a waste to have him at the end of the bench.
Hairston, on the other hand, has more skills in his tool belt. Personally, I don't buy the scenario that Hairston couldn't contribute this season. Because of his versatility, I could see him getting spot time at the SG/SF spot . Even if he doesn't, his time wouldn't be a waste as he could be sent to the D-League and called up at Pop's discretion.
Ever since the departure of SJAX, the Spurs have continuously tried to patch the wing position with a collection of veteran FAs, most of which have spurned their advances. A route that is very expensive. Or they've taken fliers on a collection of late round draftees and unproven youngsters (Marcus Williams, James White, Bobby Jones, Jeremy Richardson), all of whom have not panned out, for one reason or another. A route that is very risky.
The point is if Hairston is talented enough and even closely resembles the type of player that fits the Spurs mold and culture, they can ill-afford to jettison him. Thereby allowing another team to snatch him up. In fact, they would be damn fools if they allowed that to happen.
Whoever the Spurs elect to go with at that spot will likely be at the end of the bench anyway. At least with Hairston, he's far more versatile and could potentially solve the need for a long-term answer at the backup SG/SF spot. If the Spurs plan on remaining in contention through the waning years of the Duncan era, they can't keep up this habit of forfeiting young, talented players, in favor of the over-the-hill veteran. They've got to commit to developing the next core of players "on the fly". They've started the process with Hill. They've got to do the same at Bowen's spot. Hairston seems to fit that need. After all, they're not going to rebuild. They have to reload.