Area 1 should be configured as an NSSA when the design must allow selected external routes, such as EIGRP- originated routes, while controlling other external routes such as those from the RIP domain. A normal OSPF area receives Type 5 external LSAs, so it would not inherently block the unwanted RIP-originated external routes. A stub area blocks Type 5 external LSAs, but it also cannot contain an ASBR that injects external routes in the normal way. An NSSA is designed for this exact middle ground: it restricts external LSAs from the backbone while allowing external routes to be injected inside the area as Type 7 LSAs, which are translated by the ABR when needed. This allows Area 1 to receive the necessary internal and selected redistributed information while excluding external routes originated elsewhere. Making the routers part of area 0 would not solve the external-route filtering requirement and would weaken the intended area structure.
Therefore, NSSA provides the required balance of route visibility and external-route control. Reference topics: OSPF NSSA, Type 5 LSAs, Type 7 LSAs, external route control, area design.