I am not sure if it was changed within the last year but otherwise it does carry a cost (real multibook, not adjustment book, literal double posting) - it may also incur additional country/currency combos on top of that as mentioned. Note that the cost is also the internal cost of having to do some things several times over so automation should be a priority - but I suppose it could be replacing something even worse, but its worth noting. It also has a performance impact that everything is essentially posted to e.g. 2 books.