If you're restricting your agent search to agents who blog, you're making a big mistake.
Nah, not restricting a search at this point, and certainly not by anything so sweeping -- I don't have anything agent-ready yet, anyhow. Outside of the outlet for procrastination they can provide,
the agent blogs have two main values to me. The first is to get a more personal look at the inside of the publishing industry. I've read PW, Writer's Digest, and probably most of the other usual references in print and online. But I get a better sense of the dynamics and color, not to mention the terminology and lingo, when I see it through the eyes of one individual. It can help prevent rookie mistakes when I get a feel for how things are working, rather than a grocery list of attributes. Think of it as wanting to see in tight 3PLO what I've gotten in generic 3PO. I learned a lot from reading Chip McGregor's and Andy Zack's blogs. Doing a heap of reading online has helped me a lot in the short-fiction world, but apparently short-fiction editors and the like are more inclined to blog than agents are.
The second value is the one you're alluding to when you talk about limiting my search and seeing from blogs that you wouldn't want to work with someone -- I like to feel like I know someone I'm proposing a major venture (and possibly a lasting relationship) with. It helps me know how to approach them, and in some cases, it shows me I don't want to approach them at all. I don't have the money or time to scoot around to the conferences to meet everyone in person. A few lines on a website of what genres they take, don't take, and what they've sold lately doesn't tell me much about them as a person or what they'll be like to work with.
At the moment, it's not a big deal -- like I said, I don't have anything ready for an agent yet. But even as I write, I'm trying to do my homework on the industry as a whole, so the pieces comes out as close to ready as they can be, and I don't then have to sit with a completed novel for months, trying to figure out who's who and what they want and how to present it to them.
As it is, though, if you've got 5-7 blogs from reputable agents on your list, you're doing better than I am.