If the plan was to let ISIS form, it could turn out to be a brilliant long play.
Short term, lots of terror and chaos, Syria weakened.
Long term, could see a consolidation of Islamic States against ISIS and radicalism in general as they see the threat it poses. If that were to happen, it would go a long way to clamping down on radicalism. It won't be the US or its allies doing it, it will be Muslims doing it which is a better result for everyone.
Even if that is the plan, who the feck knows what will happen. The area is so volatile and unstable, ISIS might die and something worse could emerge.