Idealism is harmful on both the audience and the idealist
Idealists are annoying. Idealism will often have the opposite effect on what you are trying to achieve. It doesn't matter which side you have on a subject. Even on people who agree with you in general.
But it is also harmful for the idealist. It is too much about being right instead of being happy. Idealists are in conflict with the real world. Idealists also have a too simple view of the world. But the real world is really complicated. Idealism will wear you out.
The evolution of an idealist
When you are younger you are more prone to be an idealist. You have less experience and knowledge of the real world. Older people are often less idealist and more in touch with the real world. But depending on the situation it can go either way. A big factor on this is how much power you have in your life and environment. Power and idealism can feed each other. On the other hand realising your actions don't have much effect can tip you to the other extreme, and you give up completely.
Conclusion
So, what's the balance? I'm not saying not to have ambitions and purpose in life. But don't build yourself a black and white bubble. You need some interaction with the world both for the well-being of others and yourself. It can be hard. It's not easy to see the biases and blind spots of oneself.