I guess I'm just trying to gain a clear understanding of the micro expansion model. My concern is based on the traditional RTS with a story-based campaign mode and full expansion packs.
While Entrenchment doesn't offer a campaign (Sins doesn't have any story further than what is told in its opening cinematic; the rest is left for you to fill in), the amount of new content in terms of units and abilities is actually similar to what you get in a full-sized expansion pack. The starbases are very detailed and offer a lot of new strategic depth to the game, while the mines and assault cruisers were a nice touch.
units replaced or "written over" by Entrenchment units
Entrenchment adds new content, it doesn't "replace" any old content. Some abilities work slightly differently in Entrenchment than they do in the original game, but those are more subtle.
I've also read that Entrenchment adds many defensive elements to Sins. This appeals to me as I tend to be defense-oriented in other RTS games, and therefore leads me to believe I may have a better experience with those additions on my first playthrough.
Entrenchment adds mine-fields, assault (anti-structure) cruisers, and starbases. The former two are used sparingly, but starbases are critically important. Starbases do add a defensive side to the game, but a good attacker (note: the AI is not a good attacker) has so many tricks up his sleeve that they won't stop him completely.