I can certainly see why some authors might be tempted to send out cookie-cutter "Dear Agent" queries. I mean, how easy is it to find a list of emails, paste a simple letter into the text box, and then click send? Compared with all of the research and personalizing you have to do to follow individual agents' submission guidelines, option one can appear tempting. But it's not very exciting.
I love researching each agent to whom I submit my work. It's exciting to learn which authors they enjoy reading, who they work with, what projects excite them. This information is important to me. After all, if I write fantasy and sci-fi novels with a splash of mystery thrown in now and then, do I want to submit my work to someone who's only interested in historical romance? Of course not. I'd be wasting their time.
As I've said before, seeking out an agent is like looking for a business partner. You, the author, need to know that you can work with the person who represents your work. It's easier to find out in advance which agent is best suited to your needs by doing a little research. Then, make sure that the agent knows you sought them specifically. Today, for example, I spent an hour putting together a pitch for one agent. They use a detailed submission form on their website, and I didn't have everything compiled for them in their requested format. But I did my research and followed their directions.
Was it worth it? I think so! Because I take the time to research agents, I get genuinely excited about each agent I query. I mentioned a couple month ago that I read a blog post strongly recommending that authors refrain from discussing their query process on their blogs. That's because a few authors stated that they have a specific "dream agent" that wasn't the agent currently reading that post. That's not the case for me. I don't query an agent until after I'm sure that I want to work with them. Then I personalize my query to pique their interest. I'm not throwing spaghetti, here, I'm trying to craft a long-lasting business partnership.
So if you're a writer, do your research, personalize your query, and only send out one letter at a time. Not one per day, necessarily, but make sure that each letter you send works for that one agent, and only that agent.