There's a Nonymouse over in a discussion on agent pitches at writers' conferences in Agent Kristin's blog who just Does Not Get why an agent would require a completed manuscript from a newbie writer. Just does not, no matter how it's explained. I tried one more time a little while ago and hopefully that'll make the lightbulb glow, but I'm not betting on it.
This person seems to think that absolutely anybody who sits down to write a novel can finish -- that it's easy, just a matter of having the time. Maybe for them it is, I don't know. But they're completely dismissing the possibility that someone who wants to write a novel might actually find that they're unable to, or that no matter how fantastic their idea is, or how perfect and sparkly their first hundred pages might be, there's always the possibility that fifty or a hundred pages after that they'll write themselves into a corner or fall into a pit and be unable to get out. There you go -- abandoned manuscript. I think most writers have them. Heck, I have a few myself, and probably over a dozen if you count all the shorter pieces I'll likely never get back to for whatever reason.
Personally, I've yet to finish anything novel length. I'm still cranking away on my current novel-length WIP, and although it's slowed way down over the last three months or so, I'm still working on it and still have hopes of finishing. But if I do, it'll be the first time. I have other pieces which were novels in potentia but which will never be finished; I just ran into a roadblock I couldn't overcome and... there you go. I was headed in the wrong direction, or tangled things too badly to ever straighten them out in a realistic way, or got discouraged to the point where I just can't bring myself to work on them anymore. I can think of three particularly early attempts (one started when I was thirteen) where I didn't have a clear understanding of exactly what a plot was when I started writing them; one was actually a series of episodes with whatever connective verbage I banged out before thinking of something new for the characters to do, and another was a lot of characters and motivations who searched hard for a main plot but never found one.
I don't regret writing any of them, though. They represent tens of thousands of words for the bit bucket, yes -- maybe even hundreds; I don't know because I never kept a tally and I don't have most of them anymore -- but they were all valuable as practice. Every one of those dead ends, no matter how frustrating they were at the time, have contributed to making me the writer I am now. And once I start finishing novels, there's an excellent chance that the first few of those won't sell either. As painful as it is to consider the possibility about something I'm currently working on, it is a possibility, and more than that it's a likelihood. But even if I end up with a novel or two or six in the trunk, completed but unpublishable, those will be practice too, and when I eventually do make it, anything and everything I've written will have helped me get there.
Sure, you hear about a writer every now and then who sells their first novel. Sometimes it's even a bestseller. Maybe Kristin's Nonymouse will be one of those people, I don't know. But for the other 99.99% of us, getting good enough to have a novel finished and published is a process, and it includes learning how to actually finish the book. There are a lot of steps in between deciding to try this novelist thing and actually getting a novel published, and every one of them is a necessary part of that process, including learning to get all the way to the end, wrap up the story and stick the landing.