imapd automove from spool to inbox

Pardon the weird title, but I have a weird situation.

We use procmail for mail delivery via dmail, but historically did not 
trust users not to mangle procmailrc and lose their mail. So we have some 
templates, like "all mail in INBOX", and "file spam in Junk", with user's 
.procmailrc a symbolic link to the template, which belongs to root.

A little while ago I screwed up while changing a user UID, and 
accidentally changed the ownership of the template.
Later I found syslog entries like
   procmail[3774]: Suspicious rcfile "/home/foo/.procmailrc"
and realized that user's rcfiles that used that template were not being 
used (except for "foo"). But there was no outcry of missing mail - only 
one user noticed that tagged spam was getting in his inbox instead of 
being filtered to Junk.
It seems that procmail reverted to delivering mail to 
/var/spool/mail/foo.
However, when imapd ran in response to a user client, the messages were 
magically copied across back to /home/foo/INBOX (a mix folder)

This is a nice feature to have, as it turns out, but I had no idea that 
it existed. Is it a design feature ? Can I trust it ?