[Lisif Deoral]

Defending someone else's planet isn't very easy, unless you declare war on any potential enemy...

Suppose you've subjugated a neutral and placed a large fleet on his homeworld. Should any other human-controlled empire attack the homeworld without declaring war to you, your fleet will be completely useless, at least for the first turn (but I don't expect the planet to survive for a turn).

So, to effectively protect it, you would have to declare war to any allies of yours approaching the planet (in other words, "don't enter systems leading to Zymarra, or I'll break our treaty"), but this is easier said than done... Especially in crowded quadrants, many players might have good reasons to keep fleets only a turn away from the homeworld you're trying to protect.