My coach, Susan, once told me that you need two types of supporters: those who are optimistic cheerleaders and fans and those who are (as I would call them) picky pessimists.
The key is all about the timing. When your goal / idea is in the nascent stage, you need individuals who will guide, nurture, and encourage you so you can create a vision, develop resolve, and sink in roots of commitment. Once you have those things, you want to drive to excellence – and the only way to do that is to have people who will challenge, critique, and “force” you and your idea to get even better.
So there is power in pessimism if you bring it in at the right stage and time – and you don’t let the criticism discourage, dissuade, or freak you out.