The value of the city of Manti’s defensive position is underscored by the daring move the Nephite army makes. They are significantly outnumbered, and they were fleeing from what was an overwhelming army. The whole purpose was to draw that army out of the defended position so that Gid and Teomner could capture the city. This they did.
The danger was that they could take the city, but that the larger number of Nephites who were being pursued might be lost. Apparently, that was deemed an appropriate exchange, again highlighting the value of Manti’s defensive position.