If you are talking about preemptive war as political scientists understand the term, then preemptive war is absolutely a good thing.
To political scientists, the term "preemptive war" refers to a war that is undertaken when a country (Country A) knows that another country (Coutnry B) is going to attack it in the near future. If, in such a case, Country A attacks Country B first, this is preemptive war because Country A's attack was made solely because it knew that Country B was going to attack.
If you are using this definition, it is very hard to argue that preemptive war is a bad policy either morally or pragmatically. It makes no sense to simply wait and let the other side attack you. The only possible exception would be if Country A cannot prove to the world that Country B was going to attack. If Country A cannot do so, it might make sense for it to wait, absorb the first attack, and then have the "moral high ground" in world opinion.
Outside of that scenario, however, preemptive war is a good policy because a country that is about to be attacked has the moral right to strike first and because such a preemptive strike would be likely to help lead to military victory.