well I didn't say any primitives, but those in particular (
\rightskip) are bad ones to touch..
LaTeX list environments (which is almost all latex display environments, so not just
quote etc) all need to know how much space is available on the line after current indentation levels are taken into account. LaTeX has several parameters that must be updated together,
\hsize are supposed to fit. If you change
\leftskip without changing the entire LaTeX parameter structure then LaTeX won't "know" that you have indented stuff and all such environments will base their layout on the settings they think are in effect which may look OK or terrible depending on the exact circumstances, but if it looks OK it's by luck not design:-)