Mostly posts for signature purpose only. Even I am posting for signature. But some topics are out of my knowledge. Then I only read that topics and I not goes for post. I think signature post is best way to give and take knowledge.
If you cannot contribute something to the community with your knowledge then better find a topic that would let you express your idea. If you are posting in a topic that you have no idea then that would purely be for signature purpose but if you are posting about what you think about it and somehow adding knowledge for the community to know then that would be better. I prefer to add some knowledge because it is give and take anyway. I learn from you people and somehow I can share what I know.