The overwhelming obstacle here is electrical consumption. I imagine that bitcoin mining on a mobile device will be incredibly taxing on the battery of said device, rendering such an activity very hard to perform when the mobile is not docked to a charging station. If this is the case, then the mobile will be at best a USB miner that you can plug in for 8 hours at night when you sleep.
Edit: Also, it would not be unfair to assume such laborous usage of your mobile's battery will lower its life expectancy in the long run
The default is only activate on charging and only use wifi. It also waits until the battery is charged first.
It won't be mining on the CPU, only on the dedicated silicon. Difference in power usage will be an order of magnitude - perhaps 0.5W as opposed to 5W+ if using the CPU full-tilt.
It has to be gentle, you are quite right.