The purpose of the secondary seat stop should be that "when", not "if", the seat unexpectedly unlatches, the pilots seat will not move far enough back that you can't still apply full travel to all the flight and engine controls.
The problem with the fixed seat track stops is that to get the seat far enough back so that you can actually get in, you will usually be so far back that you can't apply full rudder or sometimes even reach the throttle. This is easy to test as you simply sit in the airplane and try to move all the controls with the seat at the seat position.
The beauty of the Cessna supplied system is it has an initial reel so that you can slide the seat all the way back to get in and then when you slide the seat up the reel retracts which means the seat can't slide very far back from the normal flight position.
Personally I will not fly in any Cessna SEP that does not have this mod. I also wonder at the attention to maintenance the owner of an airplane which does not have this mod, has applied to the upkeep of the aircraft. If he/she could not be arsed to get a free safety upgrade, what else are they not doing.....