I agree that in general it's rude. Context can be important. I work with one colleague sometimes hours at a time at the computer. So if a text comes, I generally don't ignore it, but I'm quick about it and often mumble some kind of apology--"Oh, just one sec, it's my mom" for example. Also, there have been times when I was in a situation where I felt really uncomfortable. These situations involve people excluding me in some way. Once I was at my brother's for a party. All his friends started arriving but he didn't introduce me to anyone and everyone ignored me. So in the end, I just fiddled with my phone.

This is just my 2 cents, as someone who does sometimes immerse herself in her phone during social occasions. But never when it's one-on-one with a friend.