I think planet skills depends on 1) finding great worlds and 2) finding and willing to use alpha cores. If sector generation does not give you nice convenient system with nice worlds, and you cannot get an alpha core quickly, then planet skills are required. On the other hand, if you get a great system and/or get extra cores (possibly keeping the one from that mission), then you can get by without them.
Me, I find them very handy if I want to set up a waystation colony so I do not need to spend so much time traveling and exploring the fringe. If I wanted to go to the red planet without my string of waystations, I probably would need two Prometheus and other stuff. If somehow along the way, I get an alert that threatens my main colony, I need to stop what I am doing and deal with it, and it is much more convenient to plop down a waystation and get supplies and stuff.
Also, having extra colony slots is nice for temporary tech mining. Suck them dry for the free items, then abandon colony when finished.
At first, I did not know having multiple colonies in one system was a great way to defend against invasions. The system with my 175% gas giant had four other planets with 150% rating and decent if not great resources. I would need to smash the Domain Nav relay and build a makeshift comm relay in its place for all-important stability and convenient intel updates. On the other hand, tying all of my colony slots in one system means fewer waystation colonies when I want to explore... or set up a military base to launch raids from.