If the primary requirement is lower power dissipation, getting a second microcontroller just to manage power may be justified, if it can switch unnecessary parts of the device off, and the savings in power exceed expenses on extra devices. (some cellular phones seem to contain a microcontroller dedicated just to power management...)

It's not only about power. When it comes to size and weight, what is bigger? A few discretes or a bulky radiator surrounded by safety zone?

Minuses: A radiator might be cheaper. And there would be fewer (though more probable - worse thermal stress) points of failure.

And about pins... Sometimes you're short on them, sometimes your design needs to be flexible and extendable. But once I found my design needs 4 binary IO pins total and maximum savings on CPU time. I didn't hesitate much to sacrifice 2 whole and half of third port to some really extreme hardware optimizations saving maybe 10 cycles per iteration...
Lost pins would be unused anyway. Gained speed meant difference between "working" and "failing".