I agree with Req but for different reasons. We hate everyone that is fighting so why not let them kill each other. Also if the Jihad is in Syria that means they can't come here to try and blow us up right?
McCain never saw a fight he didn't think we should finish so I see where he is coming from. It cements your status in the world right now if we step in and finish this. It would show Russia doesn't matter who you support we want them gone they are gone. It would show Iran and Russia just how lame your missile systems really are when operated by lame countries which most of them are at war. So it would send a message to our enemies that support Assad you can't save anyone we don't want saved.
Thats probably more how McCain thinks then some altruistic plan to spread freedom to allah lovers everywhere. We all know its always been about power. Losing it, or gaining it. For some politicians its that simple does it increase or decrease the amount of influence we have in vital regions of the world?
If we do get involved i hope we kick the **** out of Assad and make those Russian surface to air missiles look silly against a high tech attack.