I was always leery about what constitutes management traffic when going through a kernel port. Well this post from Duncan Epping over at Yellow Bricks pretty much sums it up.

The feature described as “Management traffic” does nothing more than enabling that VMkernel NIC for HA heartbeat traffic.

Much clearer when you put it like that. The vCenter Server Best Practices for Networking touches on this but it’s worded differently.

On ESXi hosts in the cluster, vSphere HA communications, by default, travel over VMkernel networks, except those marked for use with vMotion. If there is only one VMkernel network, vSphere HA shares it with vMotion, if necessary. With ESXi 4.x and ESXi, you must also explicitly enable the Management traffic checkbox for vSphere HA to use this network.