Here's my question: Once I get that proposal package on my agent's desk, do I finish the novel? Or do I move on while she tries to sell that project, and write another proposal for the next novel I want to write? I mean, I've got ideas, no problem. I just don't want to waste my time writing books she can't place. Nor do I want to become a "proposal writer," either.
Finish the novel.
Frst, if she gets an editor on the hook, you darn well better have a novel to send or you're toast.
Second, you'll only learn how to write by writing and/or you only improve by actually writing. Finishing a novel has its own particular set of problems, and the only way to master them is to actually finish. You need to finish the novel for your own professional development if nothing else.
Third, you can't ever look on doing your writing as "wasting time". Even if a book doesn't sell, it's not wasted time if you learned how to be a better writer. It's not the most productive time maybe, but it's NOT wasted. Writing is not an efficiency art. You're not Henry Ford and your book isn't coming off an assembly line. Some of the best work you do will be after "wasting time" staring into space thinking of how that body got in the armoir when the door was nailed shut.