With my old iPhone 4 I would have agreed with you wholeheartedly about the effect of bluetooth on battery life, but I now have an iPhone 6 Plus and my wife an iPhone 6 and both of us have Apple watches.
The iPhone 4 used a Bluetooth 2 chip. Starting with the iPhone 4S, the chip used was a Bluetooth 4.0 part. Bluetooth 4.0 implemented the new Bluetooth LE (Low Energy) standard, which uses a tiny fraction of the power of older Bluetooth chips. Bluetooth LE is, as the name suggests, way easier on the battery.
