A not insignificant advantage is that it is human readable, so that if we haven't set up a system yet, we can still interact with the information on an ad-hoc basis, even if we're communicating with an automated system on the other end. We often talk about system glue. Simply put, XML can be viewed as a kind of Velcro between different computer systems. XML isn't the only technical option. As we have observed, EDI, at least for the moment, is already entrenched in supply chain integration.
Efforts are underway to provide EDI over Internet technology.
And of course there are commercial products with non-XML means of communication, designed specifically for supply chain integration. However, a key difference is that ERP software built to automate businesses processes within a company. Because supply chains are made up of separate companies, integration work will likely mean getting different ERP software to communicate. Even if you are lucky enough to have trading partners who made the same technological choices that you did, because supply chains are dynamic interactions between different businesses, who may or may not be trading partners in the future, you may have to repeat the exercise if your choice of partners changes. And, of course, ERP resources are notoriously expensive and difficult to find.