The problem is how you access the disks that are sitting on a VMware server. VDDK allows you to connect and read them (in raw format, in fact) at a reasonable speed, and also understands sparseness and change block tracking.
I suspect the bigger problem is simply the fact that VDDK workflows are already defined and integrated into migration software packages. Otherwise, there are other options to copy virtual disks from ESXi hosts to other destinations that preserve sparseness and don't require shutting detaching or shutting down hosts using the disks, e.g., cloning a snapshot to an NFS server that supports sparse files.
