[Stone Mill]

If you are fighting to destroy an enemy, and cripple their ability to make war - rather than to gain territory and bolster your industrial base - then you may not need to wory about holding ground. The defender must hold ground, but you can raid systems, destroy colonies, then pull out. sometimes you cant afford to hold worlds, but you can afford to continue an offensive untill you can recolonize them later.

Other times, holding teritory is crucial, because you need their production, or their resupply depots, or need them to slow down a counter attack.

Very true. In the case where you need to hold territory, I stress the importance of adaquately defending your new gains. This may mean pushing your defensive line forward... and not a Marginot line that is static but a mobile defense that operates under the tenant of Mobility. You can't defend everything... but key points. And what you acquire is your new key point.