Otherwise it makes a point that doesn't need to be made, and is therefore boring and useless. A good thesis should probably be qualified. A qualified thesis shows your readers that you have taken into account some of the arguments of the opposition. Qualifications to a thesis don't need to be concessions to the opposition. Note, though, that this thesis implies some consideration of the perspectives of the opposition.
Nuclear weapons are horribly destructive in their effects on both humans and the environment.
But they have served as valuable deterrents to world war in a real world where not all nations will agree not to use them. Not every thesis need be qualified. However, adding qualifications to a thesis adds sophistication. It helps persuade your audience that you have considered all sides of the question, so it helps persuade even those hostile to your position that you may have an argument worth reading. Include some good thesis statements.