Believe me, I've got them. After all, my first "novel" was a college drama starring thinly disguised versions of myself and my older crush. Would I ever dream of inflicting this story on an unsuspecting reader? Lord, no. What about my second novel? Well, it was a 90,000-word talking animal epic. My mother had a copy bound for my 13th birthday, and it's the only 'published' copy that will ever exist. Do I love that book? Of course. But is it fit for public consumption? Perish the thought.
There are more - I won't force you to suffer through plot summaries. We all write them, books that offer a snapshot of the gawky adolescent phase of our writing careers. Trying to publish these books would be like trying to pass your 3rd-grade violin recital off as a masterpiece. Every respectable author should probably go through multiple trunk novels before producing one that's publishable.
So what's the point, then? Why not quit writing as soon as you realize something is destined to be a trunk novel? I'm on the brink of that realization now, struggling through a rewrite of an old novel. Personally, I believe that trunk novels are how you develop your craft as a writer. Part of the reason I love my old trunk novels is that they so clearly showcase each stage of my writing development. "Aww, it's Nora's first lengthy descriptive scene!" "Look, Nora just learned how to give a character flaws!" "Hey, some of this dialogue sounds kinda real!" They'll never be worthy of publication (if you ever see a novel for sale that says "written by Nora Coon at age 9", please direct angry letters to the fantastic agent I will have at that point in my career). But I love my trunk novels just the same.
How do you know if something's a trunk novel? First of all, if you're not writing with the hope of eventually selling your novel, it doesn't matter. Write whatever you want, and enjoy. But if you do dream of someday getting a phone call from your agent saying "Good news, Random House wants it and they're offering a mindblowing advance", read on. If you have a critique group, it's relatively simple - ask them to read it, and see what kind of feedback you get. If it's legitimate criticism, you've got something to work on. If it's a lot of people saying "I don't understand why you asked us to read a 110,000-word book about one day in the life of a talking sock", it may be time to re-evaluate. If you don't have a critique group, find a few people whose opinions on literature you trust (a creative writing professor, an articulate friend with similar taste in books) and ask them to read it.
Personally, I believe that you should finish a novel once you've started it, if only because you'll learn something from every novel you write. I am also a huge hypocrite and have about seven unfinished novels sitting on my hard drive. Most would be called trunk novels, because there are gaping flaws. Eventually, the choice is up to you: keep working, or leave it and start something else. After all, you can always dust it off in a few years, once your writing has matured, and if the idea is still good - well, that's when you yank out the plot and characters and start rewriting.