when are stats more important than att?

Main stats and weapon attack go hand-in-hand when increasing damage. Even though att tends to have a bigger impact than stats, the amount one increases your range is relative to how much you have of the other.

If you have 200 weapon attack, but only 4/4 in your main stats don't expect to do much damage; likewise having 1 att with 800 or so in your main stat won't be impressive either. For that reason you can't just say that x att = y stat.