Space division multiplexing (SDM) is a promising optical network technology with the scaling potential to overcome the possible capacity crunch problem in backbone networks. SDM enables parallel transmission of several co-propagating spatial modes in suitably designed optical fibers, such as multi-core fibers (MCFs). The MCFs are prone to excessive signal impairments due to inter-core crosstalk (XT) that occurs between adjacent cores and affects the transmission quality. Although the effect of XT has been addressed in previous networking studies by considering different approaches for its minimization, still the relation between the level of XT and network performance has not been deeply analyzed. In this work, we evaluate the impact of the worst-case inter-core crosstalk of MCFs on network performance in a spectrally-flexible SDM optical network planning scenario, in which spectral super-channels and multiple modulation formats are used. We apply a transmission reach model that account for both XT and other signal impairments and evaluate two realistic network topologies of different dimensions. The performed numerical experiments show a relatively high difference in spectrum usage and supported traffic volume in the studied networks when assuming different MCF characteristics.