Sure, but it's sort of dumb for me to bring an idea I value to the table until I have answers to all the obvious questions.

I owe it to my colleagues to not make them the bad guys by shooting down an idea.

Really depends on the context I think, brainstorming session? Naysaying does have a habbit of stunting an idea's growth in the session. Sometimes you need to imagine you've solved a bunch of hard problems before you can explore the value the idea has.

I say this as a semi-reformed naysayer. I am critical of implementation plans, but let ideas breath a bit in a more exploratory setting before I start bringing up constraints.