The pid file will allow snapinstaller to be run as fast as possible without overruns. Also it will recover from a last failed run should an older snapinstaller process no longer be running.
Avoiding overruns means that snapinstaller can be run as fast as possible, but without suffering from the performance issue described here:
This means that one can do the following
*/1 * * * * ..../bin/snappuller && ..../bin/snapinstaller
Even with a 'properly tuned' setup, there can be times where snapinstaller can suffer from overruns due to a lack of resources, or an unoptimized index using more resources etc.
currently the pid will live in /tmp ... perhaps it should be in the logs dir?