Any reason not to use '+' to concatenate two strings?

There is nothing wrong in concatenating two strings with +. Indeed it's easier to read than ''.join([a, b]).

You are right though that concatenating more than 2 strings with + is an O(n^2) operation (compared to O(n) for join) and thus becomes inefficient. However this has not to do with using a loop. Even a + b + c + ... is O(n^2), the reason being that each concatenation produces a new string.

CPython2.4 and above try to mitigate that, but it's still advisable to use join when concatenating more than 2 strings.

Plus operator is perfectly fine solution to concatenate two Python strings. But if you keep adding more than two strings (n > 25) , you might want to think something else.

''.join([a, b, c]) trick is a performance optimization.