Hmm, the roles of the current system:
1. Serves as a pacing tool for mobile military
a. Gives an incentive to get more planets so you can build more mobile military
b. Gives an incentive to keep planets so you can continue to rebuild military
2. Serves as a pacing tool for stationary defenses
a. Gives an incentive to get more planets so you can build more defenses
b. Gives an incentive to keep planets so you can continue to rebuild defenses
c. Gives an incentive to keep planets so your certain, vital existing defenses (even on different planets) continue to work (brownouts disabling forcefields and such) (This makes it such that losing power on planet A could have a major impact on the defensibility of completely distant and otherwise unrelated planet B)
3. Gives a ongoing cost to provide for this energy, thus discouraging having a huge "buffer" of energy
Hearteater makes a good point that the current m+c resource system covers 1a and 1b pretty well.
Presumably, under Hearteater's system, reactors would still have an ongoing cost, thus preserving 3.
And finally, under his proposed system (energy is local to the planet, and is for non-mobile military (and possibly fortresses)) it makes 2a irrelevant, and strengthens 2b. If you lose all energy output facilities on that planet, all the defenses on that planet that require power go down.
However, that system fails to cover 2c. Under that system, planet B no longer cares if planet A (or even all other planets) is lost, as it does not impact the defense on its planet.
Now, of course, you can make an argument that 2c is not needed, that whether you can hold planet A should have no bearing on the defensibility of otherwise unrelated planet B. (though I would not, I think its good, but others may disagree).