From our experience I don't remember at least 1 case when a hero set aim at 1 turn and used it on another. Our OL just hit the hero with order with one of his minions just to know what order is there. Rest, aim and guard should be descarded with 1 wound taken. Dungeon levels don't have lotsof areas, so no cases when a hero sets aim infront of closed door to make aimed attack on heroes next turn. Mostly we play rlt and there are several cases in wich I as a hero want to use aim. First of all it is dungeon level bosses. With average 25-30 hits and 3-5 armor it is quite hard to kill boss without power potion with shop items or early bronze. Even in full bronze every time we try to kill boss we use power potion. Sure we want not to miss with power potion, and without dodge card we can even raise a damage a little bit. Speaking about dodge, for the middle second level of a dungeon OL has this card for sure because of 3 of them in a deck. So, every dungeon level in wich we decided to kill boss and not to skip it, we use aim. Every hero who attaks. And that is more than those 1% you are speaking.
Second. Don't know about other OLs, but ours don't let us anything for free. On every chest, on every glyhp there are mosters. I won't be constructing scenarios, i'll just remember some of them.
1. Heroes are at the middle between starting glyph and one unactivated on the map. On his turn OL killed one hero (main damager) and stunned another, and put his monster on the glyph. On heroes turn we decidec that our better try would be to kill a monster on the glyph, so killed hero could immediately take action and not run from the start. One hero could aim + attack, second run to the glyph. Yes I count this situation as Need to kill for sure.
2. Main attaker (melee) just killed in the corner boss that was injured before and try to hide or run away from heroes. Trying to save tome time in turns to regroup (or sometimes regen for mana) with other summon or just to take more turns for heroes to clean the dungeon level for more cards and mana to start with on the second lvl. On his turn OL spawned 2 shades and blocked that melee atacker in the corner. That hero cannot attack shades by themselves, and only 2 other heroes could free him (range user and magic user), but one of him on the other side of a dungeon, and second one has his average damage power just on the line of shades HP to kill it. If the hero won't kill a shade in 1 turn, he will recieve 2 shots from shades back on 1-st OL turn, and all hero's party will waste 1 turn to go thurther. Same thing on the next turn. With every next turn there are more chanses to have a dodge card in OL's hand.
3. Every time we want to kill monster for sure. If we have 1 monster to kill and it not a target "for sure" then it move + attack, not battle. If we don't need to move hero then atack + guard, becuase it case of killing mob in first shot better to hit smth on guard, if mob not killed on first shot heroes could kill it on OL's turn. Actually see only one idea why better to use battle instead of aimed attack for single target: maximum possible damage isn't enough to kill a mob, but 2 average attack will do the job. As for case we have multilple targets aim and battle are not comparable.
So if we are speaking of single target we use aimed attack more often than 1% of the times (understanding that monsters HP+armor is about between average and maximum possible heroes damage).
As for vanilla, I admit, we didn't use aim too often. But we started to play descent from vanilla. And now more expirienced in game we play RLT. May be if we start vanilla again we will use aim more often, but not sure.