I say it depends on the type of trail ride you are doing and how far from home you are going. If it is short, bareback is not as big of a deal, but the farther you get away, you run into complications such as: your horse getting you all sweaty (yuk :) ), not having anywhere to carry helpful trail gear, having a lot farther to walk home in the event you do become unseated and lose your horse. :)
I tend to use a saddle for longer rides and go bareback on shorter ones.
I will say that I had one experience a few years ago where my young horses took off, and because I was bareback, I could not get the leverage to pull him into a stop...I ended up having to jump off - ouch. :) I no longer combine bareback trail with inexperienced/younger horses. :) We live and learn... :) :)