coursecontacts regularly (unnecessarily?) purging

We're still working out how best to configure all of the various cache definitions, and I've been looking at how frequently the various cache definitions are purged in a production environment.
When a cache definition is on a memcached server shared with other definitions, purging one definition will purge the others as well, as Memcached can't segregate the various definitions.

It seems the coursecontacts cache regularly purges itself, even though it seems to have fairly simple keys.

The code (/lib/coursecatlib.php @ ~713-718 in 2.8.5) looks like this:

$cacheddata = $cache->get_many(array_merge(array('basic'), array_keys($courses)));

if (...) {

    $cache->purge();

    $cache->set('basic', ...

}

It seems the keys are all course ids (and 'basic') - so purging a list of course IDs would seem to make more sense, to prevent the purge?

$cacheddata = $cache->get_many(array_merge(array('basic'), array_keys($courses)));

if (...) {

    $cache->delete_many($DB->get_fieldset_select('course', 'id', ''));

    $cache->set('basic', ...

}