In the numeric keypad, the decimal separator is automatically given by the language settings of the iPhone (in the US it is a point but in Europe it is a comma).
If I enter an expression using the comma (for example 1,2 + 3. It gives an error.
Because of this it is impossible for me to use the numeric keypad, and I have to use the regular keyboard to input the numbers and the decimal "." decimal separator, which is anoying.