You mention that there is a 5-10ms clock stabilization time, but suppose my uC sleeps for 50 ms, wakes up and works for 50 ms, then goes back to sleep. In this scenario, isn't it still advantageous to use intervallic sleeping every 50 ms?
Yes, but the benefit is less.
See that the mcu consumes 10ma at full speed and nothing in sleep.
For every 50ms, the mcu is active for 5ms (start-up time) + 0ms (for your task). Total current consumption is 5ms * 10ma = 50ma*ms.
Rather than putting it to sleep, you run the mcu at 1/1000th of its speed, no start-up time (as the mcu is always running), as in the case of a variable rc oscillator.
For every 50ms, the mcu is 1/1000th for 50ms and 0ms(for your task, at full speed). Total current consumption is 50ms * 10ma / 1000 = 0.5ma*ms.
If you want to learn more about this, TI has an application note on msp430 - with heavy emphasis on its "instant on" oscillator. That family of mcu was specifically designed for low current consumption applications.