Have those who say it's unfriendly or inhospitable not to text first never muttered 'Oh $@&, it's your mother/sister/cousin' or similar when the doorbell's rung at an inconvenient time, but opened the door with a smile rather than make the doorstepper feel bad?
Do you really want to be on the other end of that, even if you are greeted politely?
Clearly, there are two disparate schools of thought, and so long as both the dropper in and the person at home are of the same view, neither is right or wrong. It's when there is a clash of viewpoints that it matters. My husband's family are droppers in, and as I've said, I hate it. Because he happily pops around to their houses (knowing they won't mind), they feel free to do it here. To be fair, it mostly only happens when someone has a birthday, and all the younger generation have left their respective homes, so it doesn't happen as often as it did, but I still gnash my teeth under my hospitable smile, as I sweep the washing off the radiator and shut doors on my way to the kitchen, mentally calculating how I am going to finish the work I was doing before the imminent deadline. My side of the family are too far away to do it, fortunately.