Well, it depends.
In MoO, it allowed you to deny your enemies those resources, and make for easy colonization by you later. The world wasn't damaged, other then some infrastructure.
SMAC was a very easy game, compared to Civ, and the reason for that was because the AI in SMAC did not understand how to use the new stuff. That is why it was so easy for the average gamer to beat SMAC on its top level. And for beginners beat it on average. It is important to good Single Player experience that the AI understands how to use what it has available at least at a certain competence (at the average and up difficulty levels). This is because the human will eventually figure it out, and any resources wasted just drag down the AI, rather then aid it in giving the Human an obstacle to be over-come, conquered, or otherwise surmounted. For instance, the AI is still very wasteful in 1.10A with star bases... and it's really killing it's economy, which is making them be less significant in the game. All because it will build 3 starbases in a sector... one for Defense, one for Social Production bonus, and one for resource harvesting, when a human would have built just ONE to get around the wasted constructors on the base level, and avoid the 20 bc hit.
Scorched earth is a legitimate tactic. Very useful for cutting down the enemy to a more manageable size to beat apart (just piecemeal attrition tactic). Sometimes, it may just be the tactic that helps turn the tide. Othertimes, it's just fun to blow up a planet. But in GC, you cannot scorch until you are significantly up the tech tree. Heck, my medium and large normal games are over LONG before that is possible. Most are over before battleships are the main combat unit.
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