Is Entrenchment worth it? I'd say that that depends. If you play online multiplayer, you really need to have it since most people who play online multiplayer regularly have Entrenchment and you might have a hard time finding games to play in without it and you'll miss out on the 5v5 and 4v4 games.
On the other hand, if you play single player, then you need to consider your options and whether or not Entrenchment is worth $10 to you. Entrenchment adds starbases, two new ships for killing enemy structures and starbases (Vasari doesn't have one for various reasons), and a few new items on the tech tree that are unrelated to starbases.
Personally, I like Entrenchment, but I only play the game in online multiplayer. In contrast, if you don't go online and play single player, one issue might be whether or not the AI uses the new content properly--it might not add as much to the game as it does in online multiplayer if the AI cannot use it very well. If you only play single player you have options for expanding your game and for adding content to it for free. (Online multiplayer suffers from a lack of auto-download for custom maps and mods, so custom maps are rarely played and mods are never played, which doesn't mean that it isn't great fun, just not as great as it could be.) There are all sorts of custom game mods to explore, some of which are probably excellent and have a devoted fan base and active modding community, such as the Seven Deadly Sins mod that you might want to explore.
Of course, one benefit of purchashing Entrenchment is that you would be supporting Ironclad and Stardock and helping to encourage the production of PC games that lack intrusive, despicable DRM and mind-numbing consolization.