I never noticed this before as I have the habit of coding
\startalignment\stopalignment
blocks set-off with leading and trailing
blank lines for better
readability. But such practice could lead to
undesired results.
Consider the following example:
\setupwhitespace [big]
\starttext
\input
tufte
\startalignment[middle]
\input ward
\stopalignment
\input
dawkins
\stoptext
So \stopalignment implicitly imposes a
\par.
If I were to omit the blank line before \startalignment so that
no big
whitespace be included before the centered block, the tufte
text will
get middle aligned. Also, perhaps I might not wish for the
dawkins text
to be separated by a big whitespace, logically as in: