In a word- I think they're great!
If someone sees potential value in an ebook you might offer as inducement to opt in to your list, they will opt in and presumably download.
Double-whammy, dude! You just got a new subscriber plus they've downloaded an ebook with YOUR links in it! (What? you mean you didn't put your own links in there...?)
It allows you to brand yourself and build authority in your niche. "Silent salesman" and all that.
If it's good stuff, you can convert it to Kindle and sell on Amazon etc.