Dear Miss Snark,
You said that every writer has at least one novel written and hidden away. I believe you called it a learning experience. Supposing the writer realized what an oozing manure boil the script was before it was finished. Must the writer complete it to be considered a valid learning experience, by your definition?
There's a lot to be said for the discipline of finishing a novel. The pages between 80% done and 100% done are often the hardest to finish. That's one reason if someone queries me on a novel it has to be finished before I'll read so much as a sentence (this varies by agent and author but for me, with a first time novelist..no exceptions).
On the other hand if you've written yourself into a hole and there is no ladder in sight, well, you've learned something that can certainly be put to use in novel two.
No hard and fast rule here other than really try to finish if only so you know you can.