I say there are other points to consider, too. I am approaching each ms I sub differently, but ultimately what they all have in common is, I want a long-term career for my client. I am looking for my clients to be not just published, but published WELL. I am not just looking for the highest advance (though obviously those are nice) but also at publishers that have good distribution and marketing and who will be just the right fit for my client's temperment and work style, and who have a track record for selling the kind of books my client writes. I don't think that is the case with every publisher in the world, and I make my submission decisions accordingly. There is strategy here, folks. And that doesn't mean sending to "every publisher until there is not a single publisher left to send to".
And REALLY? You want to send to LOUSY publishers? You want to send to publishers that will screw you over with a terrible contract, or who will take five years to publish your book, or who can't get the book in stores? REALLY?? If I send your book to a publisher who I know has terrible distribution and you only sell 100 copies, it is going to be a lot harder to sell your future books to a bigger publisher. Things in publishing are difficult enough without borrowing trouble.
I would personally prefer to hold off for a bit if my first and second choices come up snake-eyes -- to consider the possibilities, to perhaps work with the client to revise, and/or to tweak my pitch then find different publishers later down the line or wait till the market changes somewhat, and meanwhile, hopefully, be sending out the next project. I'll give you some examples:
Client A had a novel manuscript that I tried to sell. I felt strongly that it should be at a smallish or mid-sized publisher, or a boutique imprint of a larger house, to have the best chance of success. We went a round of submissions, and everyone turned it down. She took a few months to revise the book. Meanwhile, I sent out her next project, a picture book, and sold it immediately. I sent the novel out again -- nothing. No bites. Client tweaked the book some more, and meanwhile, gave me a chapter book project, which I sold immediately. Then I sent the NEWLY revised manuscript out to some more new editors, based on new research, and with a new pitch, and sold it, finally, over a year after I started, to a wonderful editor at a great house who hadn't even been acquiring when we first started. She got a modest advance but an extremely enthusiastic editor and lots of in-house support. I am very happy that I kept my ears open for new editors, and that she didn't sit around freaking out about the first book, but rather, kept working on new stuff, and kept an open mind about revision possibilities.
Client B had a historical fiction book that I felt very strongly was highly literary and special, and that this author should be at a "literary" imprint of a top-tier house. This limited the number of people that we could send it to -- but that was OK with me, and the author. It was turned down by our top choices, who said it was too quiet for a debut. Meanwhile, she wrote an equally beautiful (but perhaps more commercially viable) second book. I sold the second book in a two-book deal to one of those top choice publishers who didn't want the first book. The first book is resting in a drawer -- maybe it will never see the light of day again... but I don't think so. Because I still believe that the book is terrific, it is just harder to "break out" with quiet and literary historical fiction. We haven't abandoned that book -- we just want what is best for it. So, when the time comes, if we still feel the book is strong, we'll tweak it and send it after her first book(s) debut and she has a name for herself.
I don't consider this giving up, or me being a lazy or lousy agent.