Well, actually it is pre-Prometheus.
It was one of the possible designs for the Prometheus project, however, the Prometheus seperated into 3 battle-ready sections, while the Legionaire class only seperates into 2 disproportionate sizes, with the saucer module having no ventral phasers, and the engineering hull having almost no crew quarters, dispite that it itself is the larger of the two.
Benefits however, is that this design is cheaper, both sections include a bridge, and the saucer module has powerful secondary warp nacelles, faster than the Prometheus. This allows one section to be manned, and the other can be abandoned. The saucer has more comfortable accomodations, while the engineering section is more battle tuned, so it depends on the situation.