Dragoons are soldiers, not duelists
The greatest asset of dragoonS ... down on enemIES
The problem with the OP's assumptions about telegraphing a leap attack is that he's considering it in a 1 on 1 context. Yes, in a duel, in a wide open arena, it would be worse than useless to perform a leap attack. However, Dragoons are not duelists or lone heroes. They are soldiers, and soldiers don't do what makes since in a 1 on 1 fight. They do what makes since on a battlefield.
In pretty much every setting that mixes melee units (legionaries, pikemen, knights, etc.) and ranged units (archers, crossbowmen, musketeers, etc), a ranged unit it best until the melee unit can close range with it. Otherwise, there would be no reason to field both kinds of units at all. This means that generals will typically try to put thier ranged units somewhere out of reach of enemy melee units, but somewhere they can still attack from. This could be at the top of a steep cliff, or on a wall, or separated by some kind of ditch or mote, or places such that your melee units can intercept enemy melee units before they can reach you ranged units.
A "jump attack" defeats all of these strategies. You can leap over walls, motes, pits, cliffs, and enemy infantry to attack the enemy ranged units directly. Even if you spend a few seconds in the air, it's still much faster than a knight on horseback could close range with enemy archers, and history tells us that when a knight has a clear path to archers, that the archers loose horribly. So, once your dragoons are inside the ranged unit's formation, their specialized melee gear puts them at a distinct advantage.
Furthermore, when talking about battle formations instead of individuals, your ranged units will not actually have a lot of room to get out of the way. In fact, when you have a crowd of ranged units trying not to get hit by a crowd of leaping dragoons, trying to dodge is worse than just standing your ground because it means all of your ranged units will be running into each other trying not to get impaled keeping anyone from effectively shooting back. So the "telegraphed" nature of the attack actually serves to disrupt the enemy formation prior to actually slamming down on thier heads. Since your dragoons have lances, even if a ranged unit gets out of the way of being crushed, the crowd is too thick to move far enough to escape the lance; so, like a heavy cavalry charge, the initial landing will impale some enemies and crush others under your boots. leaving the rest of the scattered, frightened, and confused enemies to face your superior melee weapons and armor.
Of course, this also goes both ways. Your Dragoons will likely need to sacrifice some armor and weapon-power for that power jump equipment meaning that when the heavy infantry shows up to help the ranged units, you can also jump back out of the melee to safety.