Fundamentally, we go to war with the fleet we have, not the fleet we'd like to have, and being able to build more Stingrays is more important than the benefit of extra weapons coverage. Assuming we have the time to build them.
Really hoping we made the right choice in going for three and not five extra NX-class, honestly. We voted for that because it would theoretically let us build more ships during the Earth-Romulan war, but this may not be helpful when ships take a year to build and the most dangerous time for Earth will be weathering the initial Romulan onslaught. Still, we know we'll survive somehow.