Just now, Sam F said:
But isn't using all threads all the time better than some threads or all sometimes?
If an application can use the threads and is coded to do so, it will utilize them. Having them unused for a single application doesn't do anything for performance (either to help or hurt) and other programs to utilize those extra threads in the background if they are available. If nothing is using any of the threads or cores then they basically sit idle or are "parked".