I can understand the Carling Cup, but the FA Cup should be a priority - especially to a team who are expected to win cups and haven't in the last 5 years.
I just don't understand why he would field a weakened side when it's probably their best chance of silverware (especially with Man Utd going out earlier).
First and foremost, Arsenal have a hectic schedule, playing FIVE matches between 27th December and 9th January inclusive - that's 5 matches in less than 2 weeks. Coupled with the major injuries already suffered (Fabregas, RvP, Clichy, Gibbs, Walcott, Denilson, Bendtner, Djourou etc) and Song and Eboue going to the ACoN, Wenger probably feels that he can't put his first choice XI out every week for risk of burnout. Heck, when he risked Fabregas on a rescue mission he did his hamstring in less than half an hour of play.
So, then it's a question of prioritising. The league is far more important than the FA Cup, because:
1) Securing a top 4, and preferably top 2 spot is essential for CL qualification, which is required to pay the wage bill.
2) You get an extra half a million quid for every place in the league, far more than you get for progressing to the 4th round of the FA Cup.
If anything I was surprised to see Wenger field such a strong defence.
Anyway, regarding the match I thought that was the best performance I've seen from Green in claret&blue. He only flapped once, he's certainly a lot better than a year or two ago when he looked a liability to me, like a Liverpool-era David James.