Does anyone know the impact of the above command on Cat6509 with sup720 as I am planning to enable it globally.
I will take a slightly different track than Niranjan did. If you enable the logging of state changes then it creates messages every time an interface changes. If interfaces change frequently (especially for flapping interfaces) it will generate a larger volume of messages. This may have the impact of increasing the CPU (somewhat) to process these messages. To the extent that these messages are sent to external collectors it may have the effect of increasing the traffic in the network. If there are lots of these messages it may make it more difficult to notice messages of more significance (they may tend to get hidden in all the detail of interface changes - and they might scroll more quickly accross the screen of an active monitor). The greater volume of messages may mean that errors cycle through the logging buffer more quickly. And the greater volume of messages may increase the requirement for disk storage if the messages are stored on the external collector.
I substantially agree with Niranjan that these effects are not major and are not necessarily reason not to enable the interface state change messages. But you should evaluate the impact for yourself.