Concatenation can be considered a generic operation that works for a lot of "sequential"things, as it also is in Smalltalk for example, where the character ","is used to concatenate anything that is a "sequence of things". Since "+"has a primary mathematical meaning it's IMHO quite logical to not reuse it for concatenation too - and as Araq points out - then we can also give "+"more useful mathematical meanings for sequential types.