The majority of enterprise workers carry some kind of smart phone or tablet with them. That means that folks have at least the capacity to access their data and applications if it's important to do so.
But fewer people that have mobile devices connect to their supply chain systems using these devices. It could be that they never found it necessary to do so, or that they don't want to be bothered with work issues while they are away. But I believe the issue has more to do with having the proper applications in place to easily and quickly connect to their systems. For most, I think the issue is the availability of the appropriate app.
But is there really a reason to extend access beyond the company firewall? If every transaction processes correctly, and all systems work as they should, there is little reason to access these systems. But the reality is that there are always issues to be managed.
As mobile apps become more commonplace, forward thinking EDI providers and the companies that use them are seeing the demand for these apps from their users. Even if the apps deliver low levels of functionality for status checking and minor management tasks, not having these extensions to their systems will eventually be seen as missing features.