The other day I carpooled to and from an event with a married couple who are 30 years younger than I am. I was looking forward to the drive because I'm quite fond of both the husband and the wife, and I don't get to visit with them often.
The husband drove. On the way up, the wife sat in the back and spent most of the trip on her cell phone, arranging a weekend event for their family. Because she was in the back, I could kind of shut out her end of the cell phone conversation. I didn't feel like it would be intrusive to chat with the husband so I was able to have a long conversation with him.
On the way back, the wife sat in the front seat and spent most of the trip on her cell phone with a friend she sees several times a week, talking about several issues the friend was having. I didn't feel like I should carry on a conversation with the husband, because it would intrude on the wife's cell phone conversation.
I realized the next day that I was a bit put out with the wife, who (in my view) found it more important to talk with friends she sees every week than to talk with me.
I expressed this feeling to my daughter (who also knows and likes the couple) and her view was that it was a generational difference. The wife wasn't being rude at all.
Then I ran across this article at Discover Magazine:
How many yakking people does it take to drive you freaking nuts? Not two. Not three. Researchers say it only takes one–if they’re talking to someone else you can’t hear.
So maybe it isn't rude, but it is danged distracting