What about Gauss cannon on point defence duty? Is it a waste of resources to try to defend your planet from missile bombardment? They are the cheapest of STOs, but they are still a lot. I'd think at least a half dozen would be needed, and might themselves alone deter that lone troopship.
No... it is certainly worth it.
I just recently had a colony attacked by an NPR using missiles and my planetary point shot all of their missiles down... I think I had about five batteries (4 cannons in each battery) of 35cm Plasma Cannons and 15 batteries of 12cm Railguns each with 7 railguns. They easily destroyed about 100 missiles in one salvo... I had not had any time to build up any defense stations or maintenance facility but I had had some run in to this alien before and I was worried they might try attacking the colony, so I shipped a large defensive ground to orbit defense force to the planet. It was only about 40k tons worth of defenses (roughly).
Another good thing with ground defenses are that they only cost you wealth to maintain, no extra resources. Although you don't get any resources back when you scrap them which perhaps even things out a bit in that department, depending on how often you want to upgrade them. As I play with 10-20% research that is not too often so not a big problem.
Also given that ground training facilities often are not doing that much I have no problem using that downtime producing STO defense systems and not ground troops.