As of the most recent IK Beta, your tracking type will auto swap to accommodate your tracking setup.
On Legacy when a tracker turns off, it'll freeze the IK point in it's current place. This function (which is probably just a side effect of the old IK solution) was very useful when purposefully used.
If you're going to be immobile for a while but don't want to use 3 PT, this is a very useful feature! E.g. Watching a movie while laying down; you're not going to be moving much at all so being able to lock that pose and not be forced into 3pt is very comfortable.