AbdulAlhazred
Legend
The reason we do it the way we do is simply that that seemed the smoothest reading of the rules. As a matter of gameplay I don't think it has anything special to recommend it - it produces more focus-fire on the defender, which introduces a certain dynamic into the game but I don't have any argument that it's a better dynamic than the alternative.
Well, there are some points at which it can matter. You can for instance create some Paladin builds (Hospitaler IIRC) that are devastating when you use the strict "every damage roll is an attack" definition because they can apply huge 'ignore me at your peril' punishment. That was one motive for the loose interpretation.
As for the RAW arguments, it got long and complicated and is surely not worth rehashing in these later days, but certain feats and such don't make sense unless you use the narrow definition of an attack, Explosive Wizardry comes to mind IIRC.