In this question I asked abouth the following oath
"May Jove the thundering husband of Juno bear witnes that no other Trojan but yourself shall mount those steeds, and that you shall have your will with them for ever."
It is in Book X (10) of The Illiad Hector (edition: Britannica Great Books of the Western World (The Illiad and The Odessey together), rendered into English prose by Samuel Butler).
He swears this oath to Dolon, son of Eumedes, who agrees to, in return for Peleus's chaiot and steeds when and if they should be captured, spy on the Achaeans and try discover their plans of battle for the next day.
The oath is followed by the following sentence:
The oath he swore was bootless, but it made Dolon more keen on going.
The previous question's answer establishes that a bootless oath is a useless oath. On further reading of my edition I found a second usage of bootless where it certainly meant useless so Mithrandir's answer is correct.
This time my question is why is the oath useless?
There seems to be no reason (at that point) to think Hector would go back on his word, and the horses do exist, so presuming the Trojons win (which they are at that point in the story [the Achaeans are in retreat]) there is every reason to presume that Dolon will at some point recieve the horses.
Dolon is killed on his way to spy on the Achaeans but he has no way of knowing this so that cannot be the reason the oath is useless and this would not make him keener on going...