But if that's a risk then you have a 50% chance that your shockwave will be useful or wasted. You know there's another way of putting that. Which is that there is a 50% chance of you wasting 2000 gold coins.
The leech is one of the most situational towers in the game. It's not a chance that you waste your coins or not. If you place it too close to your spawn, then your opponents can exploit its location by prematurely setting it off with cheap, durable mobs such as creepers and skeletons. If you place it too far, then it won't charge often. In both scenarios, it's a waste of 2500 coins for a level 3 leech. The best place for it is in the middle of the map where it can cover a lot of area (=> more charge) and have its shockwave affect most of the map.
The wither is considered a ground entity, not a flying one. Even then leech can affect both ground and air units from charging or its shockwave. The key to making the most out of it is to place it somewhere where its location cannot be exploited while still allowing it to charge from tougher units. As a rule of thumb, against mobs with normal speed (e.g. zombies, blazes, skeletons), it takes 2 waves for it to charge. Against very slow mobs such as cubes and creepers, it can charge completely from just 1 wave. This tower is the only tower that can deal AoE damage to air units via its shockwave.
In short, it's a support tower that is good for clearing spread-out crowds and air units. Slow tough mobs near it to give it more time to charge and strategically place the leech to make the most out of it. However, given its situational use for new players, the 2k points isn't a good investment. (Saving your points for quake, zeus, sorcerer, or necros will give you more bang for your buck.)