There's no conclusive evidence that the chemical weaponry was used by Assad.
Your deterrent relies on 3 things.
A) That everyone will be caught when using it.
B) That an attack will come
C) That the attack will cost more than is gained.
D) That the people care about your counter attack (terrorists; don't)
At best, the attack will hit 2 out of 4 points. If I was a mad violent dictator, and poison gas would get me a decisive advantage, I would use it.
And well, do you really want to let the Chemical weaponry reside in a warzone. If Assad wins, nothing bad happens. However, if the rebels win, you either get a secularish state whit it's own Fundamentalist Islamitic rebels, armed with Sarin and other poison gasses, or a fundamental islamitic state, armed with poison gas. Both don't sound that attractive to me.
Well, Putin has at least one wrong fact: there's no way it was the rebels. It had to be the government. There's two reasons why.
1: The rebels have no air forces at all, nor missiles. However, the gas was dropped from the sky.
2: 200 liters of sarin gas was used, which is way more then the rebels could have without having a dedicated military base.
Vladimir Putin is ignoring the facts in order to sound as reasonable as possible. It's political bullshitting to dress up the fact he's supporting a government that is killing it's own citizens. I don't support the idea of a war in Syria as that has the potential to go horribly wrong, but I do not agree with this at all.
1. The attack was delivered by several homemade(according to some sources) rockets. Or Mortars, according to other sources. No Planes are mentioned anywhere.
2. Fundamental Islamitic Rebels overran several chemical weapon depots a few months before the attack. They also captured several missile units.