A short guide on why you should expand and not porc in a corner.
Zero-K has two resources - Metal and Energy. Metal is extracted from points scattered across the map, and Energy is generated from the five energy structures. Think of Metal as being the flow of raw resources into your factories, and Energy being, well, energy that your buildings need to function, including your factories.
Zero-K is all about the flow of resources, not how much of it you actually have. Having zero Metal is alright, as long as the flow of Metal is sufficient for your needs. Running out of energy, however, is a big deal, as Energy is required for more than just production.
Remember the Golden Rule of Zero-K: be low on Metal and swim in Energy - never the reverse.
Why Should I Expand?
Metal is the more limited of the two resources, and so it is the most contested. Since Metal is scattered across the map, it is directly linked to territorial conquest. Though you can use Overdrive to boost your Metal production, it suffers from diminishing returns, and can't compete with expansion in the long run.
The most Metal a player has flowing in, the greater capacity for production he has. If Player 1 has more Metal flowing in than Player 2, then Player 1 can build more expensive and powerful units faster than Player 2 can.
Managed correctly (read: without wasting Metal), the flow of Metal is directly linked to the player's capacity for military action and further conquest, and so more Metal and more production. It's a virtuous cycle.
Assuming equal skill in combat management and Energy management, he who has the most Metal wins in the end.
On Turtling in a Corner ("Porcing")
Some players, having been taken in by the virtues of static defences and their higher efficiency, decide that they should sit in a corner, build defences, and get a superweapon. This goes against the rules of expansion and Metal flow, and is a losing strategy.
Again, the more Metal a player has, the more powerful his army is (assuming he hasn't wasted it). Eventually, the might of his army will exceed the strength of their opponent's defences, and they can overwhelm the defences with the numerous weapons Zero-K provides him. The defending player can't attack because he has put all his Metal into building and maintaining his defences, and so can't expand and make his Metal flow greater. This is a vicious cycle - no army means no expanding, which means no Metal and no army to expand with.
Don't be that guy.
Static defences are for very important things, like your factories and Energy generators. For a metal point, a Lotus will probably do. Your defence should be your offence - putting the pressure on your opponent.