By constantly rearming Assad, Putin ensures that the Syrian rebels will continue to need al-Qaeda to assist in their struggle against the regime thus boosting the power and influence of al-Qaeda in Syria.
As in Chechnya, Putin’s policy appears to aim at boosting the power of extremists in the opposition at the expense of more moderate groups.
So while Putin proclaims himself to be fighting Islamist extremists, his policy appears serves to strengthen them.
Such a policy is entirely consistent with Putin allowing a terrorist threat to build up which he can use to justify the implementation of oppressive security measures if and when he so desires.
In Syria it works to deter the West from intervening against Assad.