Some people keep an idea notebook because they are always forgetting things. Some do not. This is an exercise in story preservation. The bad ideas self-destruct and are forgotten. The good ideas keep floating to the top.
Stephen King advises you not to keep an idea notebook, for the reasons in the last sentence. Good ideas keep coming up to the top, bad ideas get forgotten. I've seen other authors recommend an idea notebook though. I don't see an idea notebook having an adverse effect on idea frequency. If I have one idea for a story, I'll have many more, if it's good. The more I have the better it is. The less I have, either the shorter it is, or the worse an idea it is. I guess King's reasoning is that in an idea notebook, bad ideas can be preserved forever. Well, just cause they're preserved doesn't mean you have to act on them. I, myself, am always forgetting things. In fact, today I forgot the receipt for my camera pictures. So I need to have an idea notebook. I've had many times where I've had great (read: GREAT) ideas while sleeping and I've been too lazy to write it down somewhere. Those ideas just get lost into the aether, and that drives me up the wall. So I keep an idea notebook, but usually, my notes are for one specific story. I haven't written anything for different stories for a while. I guess that's because I tend to focus on the one thing I'm working on right now, especially long works. I think it's always a good idea to write things down.