It's not so much VLAN QoS brings advantage to traffic flows, except in one case I'll touch upon shortly, it can ease configuration maintenance.
Consider you have functional purpose VLANs, and each VLAN has different QoS policy needs. If you apply the policy to the interface, and then need to change the port's VLAN, you need to also change the QoS policy. If, however, the policy is tied to the VLAN, just changing the interface's VLAN can change the QoS policy too.
As to the case where it can impact traffic flows, on the 6500 with sup720s, they only support a fixed number of interface with a policy tied to it. Using VLAN policies avoids this limitation.
"Would the policy below drop all traffic above 1gig?"
Yes and no, depends how the traffic is exceeding 1 gig. Which leads to your second question.
"What purpose bc and be have in policies?"
Interfaces always transmit at physical port speed. So, if we want to police (or shape) slower, we need to "measure" traffic volume only during some time period to see whether the traffic rate meets or not the desired effective transfer rate. The Bc and Be provide the measurement intervals in bit volume.
For example, if we wanted 1 gig limit on a 10g port, that means we only want to "average" 1 gig transfer rate over some time period. Notice the "200 ms" in the police statement. I.e. during every 200ms we'll allow what a 1 gig port would transmit during the same time.
BTW, since port rates might vary, the time period is converted to a bit volume, such as Bc or Be.
