Very useful if you use a deck based on amazoness monsters. Boosting the ATK of all amazoness monsters by 1000 allows them to beat even high level monsters. Note that they do not gain ATK when they are attacked by monsters your opponent controls in the opponent turn.
The text should state it only applies to the controller, it is indeed incorrect. (The TCG text has the same issue.) Continuous Spells can apply to both players (Machine Assembly Line does for example). Heck, Field Spells don't always do so either. (While most older ones do, a lot of newer ones don't, such as Sorcerous Spell Wall.)