Mitch makes a good point about the dangers of declaring war on abstract concepts (see original post and his comments here). I'd certainly agree when it comes to trying to right wrongs that are more or less part of the human condition. Neither poverty nor hunger, for example, are likely to ever be "defeated," no matter how many villains we think we've found. I might make a distinction for things that don't have to accepted as normal; in the "war on crime," there are true villains. We might never win the war, but by identifying the villains and taking as many off the street as possible, we secure a certain uneasy peace. But even within this category, we have to be careful. The "war" on drugs is really two separate campaigns: one to target people who sell illegal drugs and the other aimed at people we want to quit taking the illegal drugs. Those are quite separate issues, but the drug warriors don't seem to make the distinction as often as they should.
The "war on terror" is something else again. The precise point has been made by many people that this is unlike past wars with identifiable borders to secure or leaders to wrest concessions from. We likely will never know when this thing is over. Calling it the "war against terrorists" or the "war to stop Islamic extremists," or something else other than the war on terror, could obscure this point.