In fact, any undefended world within sector capital range of a potential enemy is a liability. It can quickly be redesignated as a jumpbeacon after being conquered.Protecting even marginally productive jumpship yards from gunship attack is going be extremely important
Hence, be wary of players establishing jumpbeacons or capitals near the edge of your territory. There should be no reason to do so unless they're planning to attack. If another empire (of similar size) happens to spawn right next to you, it would be best to either eliminate them quickly or attempt to forge an alliance. Keep in mind alliances, non-aggression pacts and whatnot are all informal and easily broken with no penalty, except maybe at the cost of how trustworthy you appear to other players in future.
Nebula pockets, like the one my capital Terra spawned in, are extremely strategically valuable. They provide immunity to starship attacks, and building starships within that pocket itself to defend key worlds all but eliminates any jumpship threat, unless the enemy is willing to take massive casualties they won't assault capitals and hubs in that region.
I'd use them mainly for defense, supported by hypersonics.Starfrigates (Defiance-class, at least) feel fairly impotent for their cost
*ahem* Personally, I would forcibly remove Mesophon worlds one by one from my territory. They're a liability and potential security threat. They also provide very little benefit, unless one is willing to contract arthritis buying ships in multiples of 1000 I don't think you could assemble a credible military force this way, unless your goal is harassment rather than conquest.Mesophon purchases present an interesting option for projecting force remotely