I was in the happy position of being asked for a manuscript by both an agent and an editor I'd met in the bar at a conference. (All casual talk with one little, "Are you looking for material?" in the middle of it all. Answer: yes, looking for **** if that's what you have send me a partial.)
I sent to both -- noted this in the query letter -- and heard back from the agent that this was highly unprofessional and that by sending it to an editor at the same time I sent it to said agent, I had hemmed the agent in and so I would have to look elsewhere for representation. (I am.) No surprise, this agent also is a real fan of the exclusive submission.
There are nitwits in every profession. I'm sorry to say that agents are not excluded from that statement.
If an editor asks for a partial, even in a drunken stupor in a bar answering a casually dropped "are you looking for" you send it. As you would with an agent.
The idea that you send to the agent first and let them sit around for five months gazing at their slush pile hoping it will disappear is nuts.
This is YOUR career. If someone wants to see your work (even if they asked for it in a drunkent stupor) you show it to them.
No surprise here...Miss Snark thinks exclusive submissions stink.