It is there for most of the reasons already stated.
1. The closer the battery to the APU the less cable required - to save weight.
2. The initial current draw on the battery for APU start is 1000A+. To drive that from the front of an aircraft the length of an A330 would kill aircraft main Batts pretty quickly.. It would also need bigger batteries to feed the extra power req for the start.
B737's had them to start with but were removed at operator discretion to save weight and maintenance costs. Even if the main batt takes the extra work and needs changing sooner, the overall cost benefit is worth it for an A/C of its size.
ETOPS is irrelevant as main BATTs are an emergency power source in case of all other failure. They are required for every flight for obvious reasons and can't be dispatched u/s.
BATT's don't generally do a lot of work in flight. They sit idle being babysat by the BATT chargers for emergency use when required.