As I've mentioned in other posts I'm one of those paranoid people that doesn't like systems making automatic changes to my configs.
However, the scenario postulated is valid and I've seen a similar example used for automatic provisioning of virtual servers. As I think about this I'm questioning my paranoia as I try to determine how could this process be any different than an IDS appliance blocking or rate-limiting traffic.
My paranoia is winning out. I'd rather have a scheduled script run to modify QoS policies for the expected traffic load during a given time period. This allows for some tuning of the policies while still leaving the configs and operation in a known, or at least expected, state. Trying to diagnose connectivity/latency/etc could be not so easy when the state of the traffic path is in flux.
If a reaction is needed to deal with a lack of bandwidth I'd prefer to receive an alert and then determine next steps. There may be a lack of bandwidth for a legitimate application or there may be other issues that need addressing.