I don't think anyone is saying that he should keep his place if he doesn't keep his place. The difference between saying that he should keep his place now and saying that if he plays a couple more games to a similar level he should keep is place is pretty marginal.
Ben Davies has been improving fairly consistently this season. The weaknesses you talk of were there earlier on this season when he was still adapting to the new team and (I think) working his way through a drop in form that saw him not perform up to his first season standard last season. He rightly wasn't given a chance ahead of Rose who was performing well, he responded well (going by media reports at least) and got his chance and took it.
With his inclusion and improvement in form we've looked more sold as a team too. This probably isn't all down to him, but worth keeping in mind.
Now Rose is working his way back from injury, even when he is fully fit physically he won't be fully match fit. So the idea of a fully fit Rose coming back isn't necessarily even an option. For me the clincher is that Ben Davies is 21 whilst Danny Rose is 24, I think that makes it likely that Davies has a higher potential and I would rather give him game time than Rose if they're performing at a similar level. I agree though that the competition is good for us, and having slightly different left backs to choose from can be a strength for us. Quality backup surely is a good thing and we should have games enough to give both players a chance.
Had Rose been fit I would have considered playing him against Palace just to give Ben Davies a bit of a rest and also inject the energy that Rose does bring to our team.