This is an interesting thread. I completely get what Phil is saying, but at the same time I personally do not feel obliged to reply just because someone has texted me. Its the same for email and other channels too. There is nothing special about text. I used to get more than 50 emails a day in the job I was in. At first I would waste a huge amount of time responding to each of them. What I learned over time was so many of them became irrelevant after a couple of days (ie I could have spent time replying/doing something but after a few of days the situation resolved itself and I didn't need to do anything).

In my personal life its the same. I have learned that I do not have to be at the beck and call of my phone. If I am doing something else (even if its not important) I don't have to stop and get my phone every time a message comes in. And when I look at it later I will make a call as to whether I respond or not (usually I do).

Ultimately I can understand why people may think this is rude, but that's to do with their expectations, its not actually my responsibility to meet their expectations. Communication is so easy today that a message can be fired off in seconds with no thought about whether it is important, whether the receiver should spend time on it, whether a reply is necessary. If I had a promise from someone that they would only message if it was important, then I could commit to always replying promptly. Without that I will set my own priorities and boundaries.