The problem is the range of possible splines. With a 3point spline you can blend between some shapes which you can use for instances.
If your spline have 4, 5 or more points, it´s nearly impossible to handle it. The range of possible forms is to big.
So every deformed shape will be unique and this will increase the polycount.
The resulting mesh will also use the same amount of vram.
Instances are good for things which aren´t deformed, which can be placed with one position and not realy need a spline.
Short fur, a stonewall or rocky desert, Legos, trees, grass, particles or an alphabet soup would be good.
Things where you mix some shapes and it looks nice
face