The problem with that is that it makes code like $foo} valid, so

I see what you mean (about making it harder to detect problems with braces), but on the other hand $foo} *is* potentially valid code.

For example, here is the definition that originally prompted me to make the change:

%% Centred math environment, spilling into the margins if needed
\newenvironment{mspill}{%
        \vskip\abovedisplayskip\hbox to \textwidth\bgroup\hss%
        \hbox\bgroup$\displaystyle
}{%
        $\egroup\hss\egroup\vskip\belowdisplayskip\noindent\ignorespacesafterend
}

Robin