TBeholder
Explorer
You answered your own question.Why not simply use the defender's AC as a modifier to the dice roll itself? No need to recalculate your THAC0 that way, and you don't use any inverse values, so to speak. Mathematically it's the same result, but sees to me to be more intuitive.
If the DM doesn't want to reveal the enemy's AC, the player can simply roll the dice and compare it to his THAC0, giving a number indicating what AC has been hit. For example, if the player with the 15 THAC0 rolls a 17, he announces he has hit AC 2 or better.
Practically, when it works this way, pre-calculations become useless back-and-forth. Which is why Hackmaster and Castles&Crusades remove this atavism altogether and use roll-over.
Tables that are a result of a tactical/skirmish tabletop system outgrowing its original mechanics (there was a similar thing in Warhammer too IIRC), and then already barely adequate mechanics being exported.The DMG mention is disingenuous. The 1e DMG has tables cross-referencing class levels (or hit dice for monsters) with target AC.
An atavism dragged along past the point when it ceased to give any benefit.