When people try to proselytize me in the street, I find it not only annoying, but insulting. It's like saying, "since you're not capable of doing your own thinking, I'm telling you what the Truth really is." The relationship to automatic messages is this. Here's the typical DM when you follow someone:

"Hey $screen_name, great to connect! You might enjoy reading my blog, too at http://douchebagblog.com"

So basically, this says, "You followed me, but I guess you're not sufficiently competent to have looked at my Twitter profile for this URL and other info, so I'm going to send you a link, which you may also get by email."

Twitter mentions that "Many Twitter users receive email notifications when someone follows their account or check out the profiles of new followers to see if they share common interests." This isn't practical if your account is very active so often people automate replies to follows. Occasionally, one comes through that is original enough to actually evoke a laugh or some admiration, but most are dismally lame or even spammy.

The solution? You aren't too busy to actually write to a follower you care about to thank them personally, are you? And for those who aren't of particular interest, there's no need to thank them or comment. Do your research, it will pay off. And stop with the auto DMs!