I have a friend who feels inspired every time she suggests an idea to her manager. Almost without exception, the manager will tell her that she should try out the idea. By saying this, the manager does two things: she shows that she trusts my friend, and directs her towards concrete action.
It costs nothing for the manager to tell my friend to go try things out. Because here are some things that this manager isn't saying to my friend: She's not saying that the idea will be a success. She's not saying that she will do all she can to help my friend. She's not saying that she will promote the idea to other people.
Instead, she is communicating to my friend that the best way to find out if an idea will work is through trial-and-error.
Maybe my friend's ideas aren't always pure gold. None of us have only good ideas. But that fact doesn't stop the manager from cultivating a culture of autonomy, creativity and iteration.
You can't always tell if an idea is bad
You don't have to believe in an idea 100% before encouraging someone to act on it.
First of all, we all have to learn some stuff the hard way. Most of us have to experience working with bad ideas before we learn how to pick out the good ones that are worth our time and resources.
Therefore, maybe you shouldn't always prevent people from wasting their time with a couple of flawed projects. Yes, your co-worker or employee might end up spending their time on an idea that was bad from the get-go. But they will learn some important lessons along the way. Even their next ideas might still be kinda crummy. But at some point their work starts to become exceptional because they have kept on trying out different things in the real world instead of just drawing up plans and pitching ideas.
Secondly, you might be the wrong person to judge the idea. Maybe someone pitches you a solution to a problem that you don't experience. Does it mean that there is no demand for the solution? No. It means that you are not part of the target market.
Every idea, solution, or product isn't for everyone. In fact, specific and focused solutions trump generic ones. In marketing, having sharp edges is better than playing it safe and trying to please everyone.
Responding in a more productive way
I have catched myself criticizing ideas that don't really need my criticism. I thought I was helping when I was actually discouraging people or just being annoying.
These days, this is how I try to remember to construct my response to ideas that aren't made for me: I can criticize and point out possible problems but at some point I have to show a thumbs-up for the idea. I won't promise that I will be a customer or a strong proponent for the idea but at the end of my response I will urge the person to take concrete steps towards turning the idea into a reality. I imagine pointing my index finger towards the door and saying "go get 'em tiger!"
First a thumbs-up, then a pointing gesture and a word of encouragement. In most cases, it's the best advice I can give.