[gnicholas]

Yeah, they're not allowed to get into your house at any time...but if they remember the code and then come back later they can disarm your alarm system after breaking in. Maybe you get an alert or a call from your alarm company, but if they can disable the loud ringing then they can make off with a bunch of stuff before anyone shows up (and without the neighbors thinking anything is awry, since they wouldn't hear anything abnormal).

edit: I'm curious to know whether the people downvoting this comment have an alarm system and are happy to give out their alarm code, or if they're people who do not have alarm system and just think that, at a glance, this seems paranoid. I have an alarm system and have never given out the code to anyone. We've thought about giving it to some people whom we trust, but after we thought about it decided against.