One big argument against destabilizing a dictator is you create a vacuum that can be exploited by terrorists. It's been employed by Rand Paul and Gary Johnson hints at it here. There are other arguments against militarily engaging Daesh but that one doesn't apply.
The issue with intervening against Daesh is not destabilization (unlike prospective intervention against Assad), but an issue of futility. For those short of memory, US forces have twice overthrown a dreadful Sunni-dominated state in Mesopotamia, only to have it reform as soon as they withdraw. What is to prevent such a thing from happening again once US forces expend more blood and treasure to temporarily achieve the 2004/2007 status quo? There are means of enacting a permanent settlement, but liberal democracies lack the political will to use such draconian tactics. Better to let the Russians, who are less squeamish about using traditional methods of warfare, to do the dirty work, even if they are more concerned with securing Assad then fighting Daesh (the lady doth protest too much)