Scouting is important.
I think it's just a limitation of the current state of the AI.
There are planetary "gates" planned as well to allow the mass transit of units.
Ignoring the post above me for the sake of discussion. 1. As was said, there will be ways to kill planet-turtlers coming. But above and beyond that,...
I'm pretty sure a fix (partial or otherwise) to metal planet pathing is super imminent.
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