Nice thing about BLE is that it can be really cheap and stateless, you don't need to keep polling. If it was sending each and every step count to the watch (or even every minute) that would be one thing, but a semi-daily sync for time and steps could easily fit in one BLE packet.

My little temperature and humidity sensors that show an LCD 2x the surface area of a watch and broadcast a BLE packet every minute lasts about 12 months on a CR2032 (~200mah). A CR2016 is ~90mah little less than half. And receiving a BLE packet takes way less energy than sending.

Then you have the on-demand stuff that costs a packet or two, like the lost phone finder -- which is what I'm most excited about.