Yes book not novel. While I point out that a novella is a novel and thus would be the first novel published. However, I did mean to say book. I rarely proof read myself when posting on a forum, but I should change my habits at least for this forum.
Not only that, it helps you develop good writing habits. If you're only careful about spelling, punctuation,, clarity, and so on when you are "writing seriously", you'll have to pay much more attention to those aspects when you get down to business. That means you have diverted more of your focus away from the more demanding aspects of writing.
That's a great point, to use every time you write as an experience to build your skill set regardless of what you're writing.
I've been there several times too, and I think it's like what Maia said about the ideas not being developed enough, but in my case it's also another factor, and that is I find after a while that the story doesn't turned out to be the story I "saw" in my head, and therefor I kind of lose interest in it. Or the tone is wrong or there is something in me that realize that it's wrong. It happens around the 25-30 K mark for me too, in fact. I don't know if that is because I often find that the beginning is the easiest to write and if the story isn't developed enough I can easily go on until then before realizing that middle part is an unknown territory or that the plot has some severe problems.
one way to avoid that happening is to do up a bullet point list of plot elements [main events] that you can think of taking place in the story idea as soon as you 'see it in your head' and before you start writing... that way, you'll have somewhere to go when you get to that 25-30k mark...