Well, it really depends on how much you're willing to do yourself and how much you leave to the AI. As long as you remember to check on what you Ai is doing and adjust it's orders you can focus on the important things and leave the less interesting stuff aside.

That's why you would have the terraforming ships, ones with the ability to make a planet habitable again (after an extended period of time). Turning planets into useless balls of rock, like nightraid said, could open up some interesting strategic plays in later game, possibly destroying some planets to not allow an enemy to build in a sector, making an area of dead space, or cutting them off from vital resources, assuming different planets give different bonus type things. Also, it should really be easy to completely make a planet uninhabitable, that'd most likely be a late game thing.

well this is not th real world and if there is much micromangement it will be boaring.So planets are bascly ship builders.

While being able to destroy planets might add another strategic element it would also severely change the balance of the game in favour of those who have the technology to do so and drastically reduces the chances of those on the receiving end to fight back. And wiping out entire systems would seem like serious overkill, but that's just me.

If i'm playing an aggressive game, it would be much easier to build up a strong empire and then simply destroy the rest of the habitable planets on the map, giving opponents nowhere to run. I think planet devastation or even destruction in the late game would be a refreshing change of pace and add an extra element of fear. Perhaps even the ability to cause a system star to go nova and destroy the system. It would include a new strategic element to the game. Instead of having just planetary defenses, I would like to see the ability to include system defenses such as massive outposts or long-range cannons. Sins does a good job of this on the planetary level, but it would be awesome to see fighters scramble across an entire system.

Orbital defenses and ground troops are certainly feasible, ES wouldn't be the first game to implement these elements. However, destroying or reducing a planet to a barren wasteland would generally be counterproductive, as it would bar you from taking control of that planet and using it's resources for your own civilization. Sure, the same would be true for other groups, but normally people want some kind of reward for conquering an enemy planet, and usually that means being able to use that planet yourself.