I've just upgraded to MikTeX 2.9, and find that biblatex under the natbib=true and style=authoryear options now interprets the \citet{} command to place a comma between authors' names and the bracketed year.
This was not the case under my previous MikTeX.
Here's a MWE:
\documentclass{article}
\usepackage[natbib=true,style=authoryear,backend=bibtex]{biblatex}
\addbibresource{bib.bib}
\begin{document}
From the outset, \citet{mar-84} gives the end away.
\printbibliography
\end{document}
where the contents of 'bib.bib' include
@BOOK{mar-84,
author = {Gabriel Garcia Marquez},
title = {Cien a\~{n}os de soledad},
year = 1984,
publisher = {Catedra}
}
I'd be grateful to hear of solutions or workarounds.
biblatex3.3, it is already addressed in the development version 3.4 (not on CTAN yet), see issue #373 at thebiblatexbug tracker – moewe Apr 22 '16 at 14:37