These are just the default settings.
I would imagine that MS research their defaults and set them to what they reckon will be the most commonly used.
Personally speaking I am happy that the Data defaults to Count rather than Sum. I guess it just depends on your requirements
blank needs to be included - so that if there are blanks in the data set being pivotted then they can be counted.
If it wasn't there - it would cause confusion as the total of count would not match the number of rows in the data set.