"In the case of a 97% vs. 3% battery, the call should be exactly as it is currently. Who is going to be the more lucrative for the driver."

Well, I disagree. I think they should optimize for Life Time Value (LTV) of Customer. So If a customer (with 3% battery level ) has been a uber user for more than say six months and have no history of cancellation and likely to be using it for next six months . He should be given prefrence over the one with less Life time value .