Actually, most web services are described with WSDL these days, which for the most part used XML Schema (XSD). If a complex-type is defined as being a sequence, and not all, then they are required to be in that order. All has the limitation that each element may appear at most one time, which means that you have to use a bunch of other elements that act as arrays, and makes the XSD much more complex.

More and more clients are being written by people who have no control over the server, and so, suggesting that the server be re-written is not practical. It may only have a published API that the programmer is trying to comply with, and they have no idea how it is actually implemented.