It would make 0 sense for Talon to use energy.
Energy Users are all linked with one another, being Ionian and 4 of them having been part of the same squad and under the same teacher.
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is a Noxian shmo off the streets who was just very good at his job, so much so that High command sent people after his ass just to force him to join.
If anything, one could argue why he has mana costs at all, not whether he uses energy or not.
Gameplaywise it would also make Talon broken, as one of his intended weaknesses in lane is his Mana, as he can only W for so long.
It would make laning against him as cancerous as Zed, as he'd be able to CS freely and unabashedly, never have to go B since he can last hit with Q for sustain and all-in you whenever he happens to land 2 hits of W instead of having to try and commit to doing so.
Lastly, it would needlessly gate his actual play pattern to much as he would not have the freedom to cast spells out of fear of going OoE all the time, when him having said freedom was one of the main goals of his rework.