Wonders a snarkling:
My first novel was published by a small press in 2003 (no, I didn't have to pay for the privilege!). It's gone into its second printing, and, considering that it's a niche novel (gay fiction), it's doing well.
If I approach an agent to represent me for novel #2, I assume I need a completed manuscript. Or do I? At what point in my career can I sell a novel on synopsis/outline + sample chapters alone?
The smart ass answer is: when you sell a two book deal. But that doesn't really answer your question. Most novels as you correctly point out have to be done done done before an agent looks at it or an editor will buy it. That rule applies to almost everyone.
If your career takes off and you become Philip Roth, John Irving, Janet Evanovich or John Grisham, you can write an idea on a matchbook and have a contract.
In fact, if you are a huge name author, your books are promised so far ahead you have to change publishers to write anything outside your "established" genre (Miss Evanovich) or change your name to start writing other things.
At this stage, you do have to be finished.
And for MOST novelists, you have to be finished. Second book deals require detailed outlines for a deal but those deals are subject to editorial acceptance so if they don't like what you write and think it sux...they can rescind if you don't have a smart agent who makes sure your contract protects you.
Besides you have to write it anyway, why not now?