1.
My understanding is that if Active-Active then the combined bandwidth of vmnic0,vmnic1 will be available for Vmotion & mgmt vlans.
The usage of NIC (vmnic) is actually depends on the load balancing policy which you have set for the port group. It generally wont combine the traffic, based on the policy set the appropriate nic will be used. In short only one nic will be used for a traffic at a time, policy will decide which nic can be used for a particular traffic. Following article gives information on load balancing policies
vSphere Documentation Center
2.If Active-standby , then we will be under utilising the bandwidth of vmnic used for management as traffic will be relatively low for mgmt.
Here, load balancing wont come into picture as it has a single active adapter. This configuration helps for availability i.e when active adapter fails standby adapter will start sending the traffic.