1. If you destroy their planets whenever you can, they will destroy your planets whenever they can. Not-Really-Almost Mutually Assured Destruction. I remember Paradox saying in the GamesCom that they didn't like the "Occupy planet, it is now instantly yours" mechanics of 4X games because they believed that "the rule of law would still exist in the future/in space". And for the same reason it seems plausible that many of the space entities would agree not to completely wreck each others planets with nuclear weapons and such, for the sake of polite future relations and a civilised galaxy. So not only would it give you a lot of "badboy", it would also give them a justification to use the same methods against your planets.
2. If you don't have ground troops, who will stop enemies from simply landing on your planets with their own ground troops? Or, in the dystopic future, stop your population from starting a revolution against your benevolent rule?
3. Blowing a planet into rubble is one thing, but if Stalingrad (and to some degree, Leningrad) taught us anything it is that even when every single building is destroyed and levelled to the ground... The people will still survive, somehow. Future anti-radiation treatment/protection is probably pretty good, and the bunkers could be built underground. It would be pretty much impossible to kill them from afar once they are dug in, unless you are seriously just interested in destroying the planet. Guerilla warfare would be gruesome on a planetary scale.