Whenever I can, I try to draw shapes using the pencil tool. Sometimes I modify shapes that I make with this tool. The most frustrating thing about is unpredictability with what remains after you draw a shape.
For example, try this. Create a circle with the circle tool. Then convert that circle to a shape.
Switch to the pencil tool.
Now move the pencil tool close enough to the top edge of the circle so that the cursor changes and you know that what you do will affect the shape.
Draw across the circle so that you reach the opposite side on the bottom. The line you make should be about the diameter of the circle.
Before you complete this action, which side should disappear, the left or the right? In my case the right side disappeared. The same thing happens when I draw from bottom to top.
How does one control which side disappears? Also sometimes I am drawing to add to a shape and all I am left with is the part that I added as the whole shape disappears.
Altering the shape with the pencil tool is unpredictable. It does what I want a good portion of the time. The rest of the time, I am constantly hitting undo as I cannot modify the shape to what I want with the pencil.
This wouldn't happen with an eraser. With an eraser, you know what should disappear. Also with a blob brush that adds to the shape, you know what it is supposed to do.
Having these tools would really make a difference in my work process. I could cut and paste less from Illustrator and Expression.