From looking at the monster stats in the 13 Age rule book, it seems that most of the time AC is higher than the higher of PD and MD. In D&D 4e, this was balanced by the fact that weapons had proficiency bonuses of at least +2 or +3, so wizards targeting non AC defences would have a similar chance to hit as a barbarian would targeting AC (excluding situational modifiers). In 13th Age, there are no weapon proficiency bonuses, so we have found that the fighter in our group tends to miss a lot more than the wizard or bard or druid...
Are we missing something here? Is there a bonus that the fighter should be getting regularly in combat that would increase his to hit bonus, e.g. Something similar to flanking in 4e? Otherwise the math seems to be a bit wonky, and not a lot of fun for the fighter (or anyone else relying on basic attacks, for that matter).