I saw this kind of penalty in Lineage 2. I guess facing such penalty, the player will think twice before leaving a guild/he must have good reason to do so. Also if a player was a trusted member of his former guild and he stole a valueable items from guild stash, it will give the leader certain time to warn other guild leaders - "this fellow is a robber, beware of him!" - something like that. In general it will help building good relationships between guild members and making online friends, which will be of vital importance during future guild wars.