why not fit GPS spot locators in the tail of aircraft?
Like this one?
This just serves to demonstrate that anything with an energy source could be hazardous. An energy source that the crew cannot disconnect, even more so.
This (787 ELT) was not jettisonable. If it was, knowledgeable hijackers could simply ditch it and change course. On the other hand, the act of ditching the ELT would activate it, raising an alarm that
something went wrong at this location.
The principle argument against jettisonable ELTs is their design, certification and maintenance cost. Ensuring that a part which could intentionally come loose but not damage the airframe is no small engineering task.