"It seems to me that the way to track the players is just to get a count at the cage when the winner cashes out"
That won't tell you squat.

The cage has nothing to with player tracking. The table scaners are tracking average bets by players. Average bet = comps.

The cage tracking is keeping an inventory of chips in the cage and in each table rack.
The floor man has an exact count of how many chips and the denoms of all chips in the table rack at all times.
At the end of the day the cage knows there are X number of chips missing. They don't know Paul Sax took them.

Mind Play does the same, only better. Looks like MP is coming to the Palms.