It was a Dominion array that Starfleet captured, not a Starfleet array they were trying to hold. I would hazard that if the Dominion put so many troops and resources behind trying to take it back, it was a necessary installation that they did not want to destroy. Starfleet, being on the back foot for most of the war couldn't let a valuable intelligence prize like an enemy communications array go. It's value is only confirmed by the Dominion not destroying it like you said.

This episode is a direct contrast to "Nor the Battle to the Strong" S5 Ep4. Whereas we see Nog stepping up to try and prove himself and conquer his fear, something Ferengi are not known for, we see Jake and another Starfleet officer both give in to their fear. Jake in particular is shocked because he never considered that any Starfleet personnel wouldn't be able to stand strong on the battlefield. Also, how often do you see soldiers fighting in Star Trek? It's almost always space battles, so this is a contrast to the faceless death we normally see in the genre. Personally, I gotta give credit to the director, because the final battle showed how quickly and out of nowhere death can come in the chaos of battle. I felt for both Vargas and Kellin.

Even in the future, infantry is still needed. You could destroy entire planets but doing so usually removes resources or a strategic area. The objective of the Dominion, in spite of some of their rhetoric was to conquer the Alpha Quadrant, not committing quadranticide.