[Will]

You're assuming that your empire has a large excess of resources. To take from your examples, if you build the fleet the "standard" way, the full 100% cost will be spread out over 5 turns. IIRC, a component costs 20% more to retrofit into a design, so you end up paying 10% of the cost on top of the original design if you have 50% cost shells, and this cost is spread out across 3 turns (about 25% + 25% + 60%). If you're strapped for resources, or running a deficit, then this strategy could push you into negative resources, and cause random scraps. So, it's only viable if you have lots of excess resources, and want to spend around 15%-20% more (retrofit costs, repair base costs, extra maintainence) to get ships finished a few turns faster. Better than the Emergency build option for many cases though.