There are several drawbacks to the use of the word "ISIS" or "Islamic State" to refer to the terrorist organisation otherwise known as "Daesh".
Firstly, "Isis" is a common name, used both for other organisations and even to name children. By associating the name with such a cruel entity like Daesh, one is causing unnecessary harm to a large group of people.
Secondly, "ISIS" or "Islamic State" contains an explicit mention of the word Islam. This has two negative drawbacks: firstly, it undermines the largely islamic effort to counter Daesh's activities, along with the sacrifices made by the largely islamic victims of terrorism.
And secondly, it serves to strengthen Daesh, as they can thus more easily present themselves and their ideas as a true interpretation of Islam and thus attract more muslims to their cause .... even if their interpretation is the correct one, surely we should do them no favors by giving them the name they so desire, especially considering the fact that it isn't at all commonly accepted by other muslims that their interpretation is correct. By naming them "Islamic State", one is however sadly sending the signal to young and easily manipulated muslims that this movement is where they belong. In short: the name itself is a propaganda tool, and we shouldn't grant them that tool.
Finally, Daesh themselves hate to be called Daesh, so that is in and itself a reason why we should be doing it. After all, they must not like that word for a particular reason, and whatever that reason may be, we should exploit it.
So, why don't more countries in the western world, such as most of western europe and USA, not use the word Daesh, rather than the more commonly accepted "ISIS" and "Islamic State" (the latter is even a tag on this very site)?
NOTE: I am well aware that sometimes such things are uncontrollable and take a life of their own. But, that does not mean that a politician or media person can't take a responsable decision and say "you know what, from now on I will be calling it Daesh. Let's force a change". This is what France has attempted.