While not presented as an opposed check, when tumbling, one usually is maneuvering past an opponent who, presumably, would like to make an attack against you. That seems to fit better as an opposed check (with a variable difficulty depending on the opponent) than a static check.
While I see the merit to that position, does it not suggest Concentration to cast a spell defensively ought also to be an opposed roll? Is the roll to determine whether you are able to avoid the target's attack (such that a more skilled opponent is more likely to get that attack in), or to avoid presenting a special opportunity for him to take that attack (flat DC)? Maybe there ought to be an opposed roll before it is assumed my action dropped my defenses to even allow my attacker a free shot.