Then again, there have been many instances (most recently during Hurricane Irene) where landline service went out but cell networks were mostly intact.

Redundancy always gives you another level of assurance, but it's not like landlines are necessarily more reliable than cell service. It is good to have a solar charger and/or battery backup - really, it's good to have more comprehensive backup home power (phone service doesn't seem so important if you don't have heat or water).