Logic Behind Using Chemical Weapons

One the blog, Political Violence @ a Glance, Barbara Walter gives a Friday Puzzler.  Today the puzzler is:

Given Assad’s extensive conventional weapons capabilities, why use chemical weapons? What’s the logic behind using them?

My first thought on the matter is signalling.  Non-conventional weapons use sends a signal to the opponent targeted by the weapons use.  The signal here is that the massive conventional arsenal that will destroy urban centers does not have to be used.  Weapons of mass human destructive capacity can be used instead to kill and seriously incapacitate opposition members.  The second part of the signal to the opposition is that Assad has the will to use internationally unpopular weapons, not being deterred by statements from international leaders about the unacceptability of using chemical weapons.    Of course, the second part of the signal also has some consequences, namely what will the international community at which Assad thumbed his nose do as a response to the use of chemical weapons? Of course, I am also not considering here the rationality of the decision to send the signal.