The String Object class makes it easier (for some, not me) to write string manipulation code without knowing what is going on in memory.
What do you think happens when you concatenate two strings? Not caring is for people who have excess RAM. How can that concatenation be done in read-only flash? Are you going to write a function for "you can't get there from here"?
I'm not the least bit surprised when String users have memory problems. They don't C so well.