Here is a short explanation of what an EOL zone (providing the reisitor is at the END of the Line and not in the panel) will give you. It will setup a voltage between a range
that will register a non-violated zone. For the Elk this zone will be somewhere between 4 and 8 volts (may not be exactly that, but you will get the idea).
When the zone opens (door/window opened for instance) the voltage will be above 8 volts and register as a violated zone. If someone tries to thwart the system by placing a "short" (via wire jumper) on the wiring between the panel and the sensor the voltage will drop below 4 volts and register as a violation (again as long as the resistor is AT the sensor).
Also, if the wiring between the panel and the sensor/resistor shorts out, the voltage will drop below 4 volts and the panel will register this as a violation.
For the non-EOL zone there is no zone
, only very high and (near) zero volts. When the voltage crosses a threshold (I think it's still eight volts in the Elk) the zone will show violated. Anything less than this voltage will show normal. If the wiring shorts out you will never know it because there is no "lower" voltage threshold as in the EOL zone.
So if one would not expect a problem with the wiring (which shouldn't happen using proper installation techniques) and they don't want to protect against someone chipping out a section of their walls to try to thart the system (short wire between panel and sensor), you (in my non-professional opinion) don't need EOL zones.
One advantage an EOL zone gives you is a larger voltage range between violated and secured zone (since there is no lower window) and would be more immune to any noise in the system.
I hope the above was explained correctly, if not, please let know so I can edit/correct it.