# Difference between \newcommand and \pgfmathsetmacro used in TikZ \coordinate

Consider the following code:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\newcommand{\MA}{4}
\pgfmathsetmacro{\MB}{4};
\coordinate (V11) at (0,0);
\coordinate (V44) at (4,4);
\draw[black] (V11) rectangle (V\MA\MA);
\end{tikzpicture}
\end{document}

This works. However changing the \draw command to:

\draw[black] (V11) rectangle (V\MB\MB);

produces the following error:

! Package pgf Error: No shape named V4 is known .

Why is \newcommand working here, when \pgfmathsetmacro fails?

-
Related Question: What is the preferred way of defining a TikZ constant?. –  Peter Grill Jan 21 '13 at 23:28