well Dale-- When a hubber responds to a comment you leave on their hub, it lets you know that they not only received the comment, but appreciated it. Makes the reader want to continue commenting.
Also, the more you answer comments on your hubs, the more they show up in hubtivity so there is a bonus for you too, as it may draw more readers to your hubs.
I try to comment back as soon as I can. I prefer to comment to each one but if I get a lot of comments overnight and have to answer them in the morning, then it's one long answer after a group of comments, I still respond to each one, though. Include their name in my response so they know I saw their comment and appreciate it.
The blanket comments are a response, but kind of makes the commenter feel like you didn't have time to read their comment and aren't going to take time to respond.