ISIS is a little different in that they have actually acquired military hardware, have oil refineries that generate revenue, and have to set up a (de facto) country. I wouldn't call it a country, so much as they are ruling that area, so they will have some infrastructure from which they enact their plans. They will have tangible targets that bombing can eliminate, which will greatly hamper their expansion and ability to start launching attacks in other places.
You're right, we will never defeat the ideology with bombs, but we can kill a lot of crazies who are carrying out the ideology. On top of that, an allied country, Iraq, has asked for our assistance, and we are obliged to help them now.
We defeat the ideology by extricating ourselves from the affairs of that part of the world. Since that isn't feasible this week, we kind of have to do this instead. We are stuck, and bombs are the answer.
