Because it wasn't really necessary.
Swann can't keep the key in any case. If Swann would somehow get caught for interrogation after the murder (for example, because he was seen in the vicinity of the crime scene), he mustn't have any evidence on him that incriminates him, he must get away completely clean. Otherwise Swann might feel pressured to spill the beans on Tony's whole plan. There must be no provable connection between Tony and Swann.
In your scenario, Tony would have to explain to the Inspector why he keeps a third key on the ledge without telling his wife about it. He would also be asked when he had this third duplicate made, and why. In other words, creating a third key is an extra hassle and a deviation from his normal habits that would raise questions.[*]
The point of Tony's plan was that Margot would end up dead inside a locked apartment, and nobody would be any the wiser.
The keys only became a point of major issue after Swann had failed to kill Margot and ended up dead himself instead (which was, of course, not foreseen by Tony). The question about how Swann gained entry only became an issue after his dead body was found inside the apartment and his shoes had traces of the doormat at the front entrance. If there was no dead body of Swann inside the apartment, nobody would be asking "How did he get there?".
That is, in fact, where Mark searches for a spare key during his fumbling attempt to explain how the killer may have entered.
That attempt was to save Margot from the death penalty, by demonstrating reasonable doubt with the possible hypothesis that Swann was an intruder who was "supposedly" sent by, let's say,
Tony, to kill Margot. Mark's hypothesis didn't involve a third key that was kept on the ledge just in case Tony or Margot locked themselves out.
[*] In that case, the smartasses on this board would have asked: "Tony was so smart, why did he need to create a third key? Why couldn't he just simply secretly steal Margot's key from her purse, and place it under the stair carpet? She wasn't going to need her key that night anyway."
______
Keiko Matsui & Carl Anderson - "A Drop of Water"
http://youtu.be/kPUENUUuqSk
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sagaylo
7 years ago
I appreciate your points. My main thrust was to underscore that in any plan, keeping it as simple as possible is the best way to ensure success, thus the complexity of swapping keys among the participants seems a sure way to allow unforseen mistakes to crop up. I mentioned that Swann might even keep the copied key only to indicate that its presence would not be a problem since Tony would have already admitted to the police that it existed, he kept it on the doorway ledge for his own use, and that is where Swann found it - entirely without any help from Tony. Thus, the copied key would not prove any ties between the two men. I don't see why Tony would need to explain why he did not inform Margo of the spare key, after all, she always carried a spare in her purse, and the spare on the ledge was strictly for his use (foolhardy though it may have been), as he may have been caught outside without his own key several times in the past. Of course, it makes even more sense for Swann to simply replace the key on the ledge after leaving the home. Whether the plan succeeds or fails, it doesn't matter where the key is found because it's not a point of contention as far as the investigation is concerned. The issue of when Tony had the copy made also seems irrelevant, since the mere existence of the spare key would resolve the question of how entry was made by the killer. In any case, hardware stores in the 1950's did not make any sort of fuss over duplicating house keys - no records or other encumbrances on the customer would have been required.
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