Qt's QStrings can be concatenated by operator% which uses expression templates to precalculate the resulting string's size and optimize several chained calls to operator+. See this question of mine for more info.
Why hasn't std::basic_string adapted a similar construct? Is this even allowed per C++11? I see only advantages and clearly ABI compatibility can be broken by library implementors when they want to (and C++11 provided a good reason even for libstdc++).
Because nobody proposed it for the standard; unless someone proposes something, it doesn't get in. Also because it could break existing code (if they use operator+ that is).
Also, expression templates don't work well in the presence of auto. Doing something as simple as auto concat = str1 % str2; can easily be broken. Hopefully, this is an issue that C++17 will resolve via some means.