Currently, MultiBindings are:
1) Having Difficulty Reflecting a Single Data Value accurately.
2) Not able to process MathConverter Operations

Repro Steps:

1) Create a Page with a MainPage.xaml.
2) Create a DataStructure named 'MyDataStructure', containing two 'Number' Single Properties 'MyFirstProperty' and 'MySecondProperty'.
3) Create a DataSet for the DataStructure named 'MyDataSet', and fill in a value of '20' for MyFirstProperty, and '10' for MySecondProperty.
4) Apply the MyDataSet DataSet to the root of MainPage.xaml

5) Introduce a TextBlock to MainPage.xaml, and in its 'Text' Property, click on the 3-Dot menu and select 'Create MulitBinding'.

6) Click the '+' (Add icon) at the top right of the MultiBinding Window, and set its Path Property to 'MyFirstProperty'.

Observe how the TextBlock on Stage is not reflecting the MyFirstProperty Value of '20'.
Expected Results: With a StringFormat of {0} and bound to a Data Value, the TextBlock's Text Property directly and instantly reflects the Value.

7) Clear the LocalValue for the MultiBinding's 'StringFormat' field, then set 'MathConverter' as the 'Converter' Property.

Observe how the TextBlock on Stage is not reflecting the MyFirstProperty Value of '20'.
Expected Results: With a MathConverter format of {0} and bound to a Data Value, the TextBlock's Text Property directly and instantly reflects the Value.

8) Add a second Binding to the MultiBinding, by clicking the '+' (Add icon) at the top right of the MultiBinding Window, and set its Path Property to 'MySecondProperty'.

9) In the MathConverter's 'Expression' field (accessible via the MathConverter Expander), set a value of {0}-{1}.

Observe how the TextBlock on Stage is not reflecting the MyFirstProperty Value of '10' [MyFirstProperty (20) - MySecondProperty (10) = 10]
Expected Results: A MathConverter displays the result of the mathematical operation between its two Bindings.

10) Remove the 'MySecondProperty' Binding from the MultiBinding, leaving a MultiBinding with a single MyFirstProperty Binding.

11) Clear the MathConverter Property, and instead, insert '{0}' (no quotes) into the 'StringFormat' Property.

Observe how unlike like in Step 6), the value for "MyFirstProperty" actually appears this time.

12) Change the Path Property of the Binding from working 'MyFirstProperty' to 'MySecondProperty'.

Observe how the value in the Text Property, and on-Stage is not updating from 20 (MyFirstProperty) to 10 (MySecondProperty).
Expected Results: The output displayed both in the Properties, and on-Stage correctly reflects the Binding associated with it.