Indeed, this is a topic of debate inside Nimble too at the time.
Speaking with VMware engineers the consensus is that Enterprise or Enterprise Plus licensing is required for this functionality, but vSphere doesn't enforce that restriction should it find a lower license key - this is up to 5.5 u1 where we are today.
Our vSphere installation MPIO vib actually installs two services - NCM (Connection Manager) and NCS (Connection Services); whilst it may appear to be working on the front end it's unknown whether the Connection Service daemon is running as expected; so in an example of an multi-array scaleout cluster having NCS not working as we expect could be detrimental. This is where Engineering (including Wen Yu and a few others) are trying to figure this out right now.
Whether this will change in vSphere 6 is unknown and is something we're trying to clarify! Stay tuned...
Do you have an update on this?
According to the webpage below, vSphere 6.0 Standard licensing includes Virtual Volumes, but does not include Storage APIs for Array Integration, Multipathing.