Because 'This is a terrible idea' or 'I don't like it' or 'omg yes, I want it' aren't accurate.
I think the benefit of it is very small and based on a desire for something neat-o. I'm more okay with it if it ends up being something minor, and I'm less okay with it if it's something major. I'm not 'dominating' anything by referring to things as projects to undertake rather than just neat ideas or stupid ideas.
Most ideas that are presented have some sort of value that can be gleaned from them. I'm not sure what you find so out of line by trying to put my input in context as high, medium, low, or nil priority, but if it really bugs you that much I'd consider it something to PM about. In no place did I claim to assert authority or power over how staff chooses projects by saying where I think each idea could fall in terms of priority.