Next-generation elastic optical interfaces will support a wide range of line rates and modulation formats. Such transmission schemes enable doubling of the channel capacity at the expense of a lower reach, whereas a flexible DWDM grid supports the transport of multiple optical signals within a single frequency slot by packing them closely, thus saving spectral resources. The resulting multitude of options on the line side provides network planners the capability to derive the most suitable one for each individual path inside the network. However, planning and operational complexity should not be overlooked, since aspects such as added spectrum fragmentation can hamper the expected network improvements. This paper investigates how to obtain the best compromise between harnessing the main benefits of next-generation optical interfaces, while keeping the complexity of the underlying system at a reasonable level. In particular, we define a novel network spectral efficiency (SE) metric that enables, on one hand, to highlight the relevance of supporting a flexible grid to improve SE and, on the other hand, to demonstrate that the grid granularity can be based on coarser 50 GHz increments without major penalties. The effectiveness of this parameter has been verified by considering two reference network topologies. Finally, the paper also discusses how future optical interface technology developments will shape network design.