Otherwise, agents are submitting to editors just as we do to them, so the chances of them being able to shop around the ms seems pretty slim.
The difference is that agents tend to know the editors they pitch to, either professionally or by reputation--often, because they've sold to those editors before. If agents submitted to editors just as authors submitted to agents, you wouldn't really need an agent. A large part of an agent's value to an author is the agent's insider connections and knowledge.
Agents take on work they think they can sell, which means that they have to be thinking about which editors to approach while they're deciding whether or not to extend a representation offer. It makes sense that the agent might sometimes pitch an editor or two s/he thought could be right for a particular ms. before making a contract offer, to get an idea of the editors' responses and confirm her feeling that the ms. is marketable.
You don't want an agent to send your ms. round without actually having a representation agreement in hand, though. I've heard of a couple of situations where an agent did this to test the waters, and then rejected the writer when no one was prepared to make an offer. That's a horrible situation; without ever actually being represented, many of that writer's submission opportunities were eliminated.
- Victoria