Spot needs to be put somewhere where it sees the sky, something that you will forget and leave behind as your aircraft sinks underneath you.
The key to the SPOT, is that although it can be used to alert by pushing a button (no G switch), it's true value is the absence of a transmission, when the observer watching over you expects to see one. If it sinks to the bottom with the plane, it has done its job, as it left a mark on the computer of the observer, within the preceding ten minutes.
If an ELT or PLB sinks to the bottom, it is truly useless. But, yes, they both have their place...