Klara, I share the same thought as you... When I personally see a book that is "free" I wonder why it is free. A person puts a lot of effort into things and to give it away freely would make me wonder if something is wrong. I have no problem with the writer giving away limited copies for free as a promotional gig, but to have it free indefinitely in a store would be suspect to me.
With that said, I have downloaded free books before. Most of them I learned pretty quickly why they were free (either really, really really short books or poorly written). However there have been a couple that I have honestly enjoyed.
I would avoid keeping it up there for free. Possibly put an advertisement out stating that for black friday only the book will be free or for only a dollar, and then bump it back to normal prices after that weekend or that 1 day. This way, people who come late to the site will still look at the book and decide to buy it or not, helping you to generate more traffic while giving people a chance to get your work at a discount. Obviously the goal is to get enough people who are willing to take a risk and read it for free/cheap in the hopes that they enjoy it and tell their friends about it who will in turn come and buy it at full price.
Marketing is a risky thing, I say there is no issue with it as long as it isn't left freely but used as a limited time promotion.