In youth soccer it was always man defense (in the back 4), but as I got older it became more zone. I think the general idea, on the coaches end at least, was that as you get better you can handle zone d and stay with runnerst that move around. My personal opinion is that professionals can handle zone, but even for them, in certain cases (ie, when they play againts extremely dangerous players) man marking is necessary and smarter to employ. I don't necessarily think that an entire back four (or three) should man mark, but when playing against a game changing midfileder or striker it's best to put your top defender on them and attempt to mark them out of the game. I've seen too many games where someone like Kaka is given too much space because of zone defense and they just tear the defense up. If a team has a defender that is good enough, they should be able to prevent this from happening.