mesh - Using instancing for everything?

Instancing improves performance (significantly) when rendering multiple (hunders? thousands?) copies of the same mesh at once. But how much overhead does it have when rendering exactly one copy with one draw call? Would it be a good or a bad idea to use instancing for all geometry rendered by engine?

Edit: Let's say we're creating an FPS game. Most of the objects have only one instance: a knife, a gun, a machine gun, a building and a radio tower. But there are also some objects with multiple instances: trees (e.g. 3 kinds of trees with hundreds of instances), grass and so on... What I mean is: instead of rendering the one-instance objects the "traditional" way and trees and grass using instancing, we render all of them using instancing. So our radio tower has only one instance (whose information we store in an instance data buffer) and we render that tower using some kind of DrawInstanced() call with instance count equal 1. Same with all other objects (of course, trees and grass have multiple instances).

So my question is: is it a bad idea to draw a single instance of an object using instancing? Does instancing have too much of an overhead (memory- and performance-wise) or is it in any way undesired for rendering single-instance objects?