I find side reins to be a very useful tool. As other posters stated in the wrong hands they can be very harmful, look at this picture for example: http://www.vgp.com.au/brokeford/page...ollkur%20e.jpg
Obviously that's not what you're going for. When properly adjusted side reins should only come in contact with the horse's mouth when the head is in front of the vertical. Here's a picture of what I consider to be the correct adjustment, straight from the Pony Club manual: http://i289.photobucket.com/albums/l...ho/Image52.jpg
Side reins allow the horse to have something to "lean" on. They are tools for developing better balance and muscle. They are intended to simulate the actions of a rider's hands and arms. Without side reins or some other similar apparatus I feel that longeing can only be for blowing off excess energy, not for training. Side reins facilitate proper movement in a horse, with them on horses are encouraged to seek out a connection and work over their topline. That is why longeing is great for bringing a horse back into work as it allows the horse to strengthen their muscles before having to carry a rider. Since you're inexperienced, I'd recommend that you try this with an experienced person who can help you out. I'll try anything (so long as it's not blatantly harmful) once. You can't tell what will or won't work on a particular horse until you try.
I hope that helps you, looking back I realize that I've practically written a novel...sorry.