In which situation one should enable the bridge_stp for the host bridge when using KVM? Is it worth keeping it disabled when not needed?
The Spanning Tree Protocol is a technique to mitigate problems with a loop in network topology.
Unless you are absolutely sure that no loops will exist in your network, STP will come in handy. Sometimes, loops can happen due to human error and it has the potential of bringing down a network.
I've personally experienced such down-time that was caused by a simple ethernet cable between two switches.
I generally just keep STP on as the overhead is minimal and networks can get very complex as it grows.