[MootWoop]

I'm not an expert on power optimization, and I don't know how this is done in micro-controllers. But I would think that the hardware clock would be implemented using integrated analog components (capacitor + resistor circuit) rather than digital logic, to avoid repeatedly switching on and off even just a few transistors.

[BostonEnginerd]

The power needed to keep the analog comparator going will probably be higher than keeping the 32kHz clock running.

[pjc50]

Apparently external xtal is marginally best (http://www.microchip.com/forums/m341592.aspx); which makes intuitive sense, given that it's exactly the same scenario as a digital watch. 32khz crystal; driver; counter; comparator. Few hundred tiny transistors. Consumption probably less than battery self-discharge.