With the expected 6-8 encounters and 2-3 short rests per adventuring day, features like this that you can use twice and recover on a short rest are intended to be usable about once per encounter. This is further emphasized in the case of Arcane Shot by the fact that rather than getting more uses between short rests, they get a feature that allows them to recover one use of it per encounter if they run out.
For groups that don’t take a lot of short rests, it would be in the spirit of the rule to allow features like this and the warlock’s spell slots to be used once per encounter instead of twice and recovering both uses on a short rest. In fact, it’s probable that the reason they don’t just work this way by default is that encounter powers were one of those unpopular “game-y” features of 4e, so instead of giving them to us straight, they gave us short rest recovery powers, and since 5 minutes short rests were also deemed “unrealistic”, they had to give us two uses between a smaller number of longer short rests per day, to preserve the functionality of encounter powers while disguising their design.
