I don't think this is necessarily true. I am an avid reader and have my Kindle 2 for over 2 years now and think I have spent maybe $75 - $100 tops on books I have bought. Before I had my Kindle I would buy all of my books at the bookstore since I don't prefer the library. Now I find that there are lots of free offerings on Amazon - I can download them and keep them on my Kindle indefinitely until I get around to reading them. I also love the indie authors that self publish and offer VERY cheap books on Amazon as well. These are authors I wouldn't have had a chance to read without my e-reader and browsing Amazon / Kindleforums.
Cost effectiveness might be a factor to you but it is not that way for everyone who chooses to get an e-reader. I mainly got my Kindle for the ease of use, the fact that I don't have to lug my book around and physically turn pages. I don't have millions of books around my house anymore and I feel like I am saving some trees this way. The cost of the books was secondary in my decision.
That being said - I am partial to the Kindle but I admit I have never used the Nook. I like the functionality of the Kindle and the customer service of Amazon. I love the e-ink and the fact that I can read my Kindle anywhere, anytime. Get what works for you.
Sorry in my original post did not mean that the only reason someone would order a Nook is because of being able to borrow from the library. The "U" was supposed to be "I" Its the only way I would buy an ereader. I read alot of the current bestsellers and that is where I would spend tons of money if I had to buy them.