It seems to me, one difference with the landline is it is seen as serving the house, not the individual. When it rings, it is generally the case that whoever is nearest will answer it. And if, for example, it’s cousin George calling to invite us for dinner, he won’t care who answers as long as somebody does.
In contrast, I would not dream of answering other half’s mobile, or even looking to see who was calling. Nor would I look at her text messages. A mobile is, to me, a very private thing. George’s call might thus go unanswered, even though somebody was available to talk to him, just because he called the ‘wrong’ mobile.
When calling others, I always prefer to call their landlines rather than mobile. I know how embarrassing/annoying it can be when a phone starts ringing, say, at a supermarket checkout, or on crowded public transport. I don’t like to feel I might be contributing to such a nuisance.
Thus, since I feel mobiles and landlines serve quite different purposes, I want one of each.