when are stats more important than att?

Well, for physical attackers, it really does depend. Wep. attack adds more damage, but STR adds HP, DEX adds avoidability and accuracy, and LUK adds... hands. Although if you're going ___-less, Wep. Attack>Stats all the time, as you're going for damage anyway.

If you're a magician, 1 INT=1 Magic Attack, however INT adds magic defense as well. Therefore, INT>Magic Attack.

STR adds HP? Since when?
DEX adds less avoid than LUK does (IIRC DEX adds hands too).

As for INT, it might also be worth mentioning that every 10 INT adds 1 MP, so that is another reason why it is better than Magic Attack.