I saw someone mention earlier that a longarm system gives you more travel. This is NOT true. Suspension travel comes from spring/shock length. The long arms keep the suspension angles in check throughout the arc of the suspension's travel. A benefit is a better and smoother on road ride. If your current short arms are binding, maybe because of the use of rubber or urethane bushings that don't flex(like factory arms, Rough Country arms, Sky Jacker arms, etc), instead of flex joints like a johnny joint(Rock Krawler arms, Offroad Evolution arms, Teraflex arms, Full Traction arms), that is an issue with the current short arms you are specifically running, not all short arms have that problem. While switching to longarms will solve your control arm binding issue, switching to a high quality set of short arms with johnny joints or a similar flex joint will also help you take full use of your shock travel and get back flex you are losing due to control arm bind. But, like Eddie said, once you reach a certain lift height, like 4" on a TJ, or say 5" on a JK, those short arms will have a pretty steep angle to them, and at that point a longarm swap would help you greatly. Below those heights, quality short arms will do great.
I've seen long arm kits that use crap bushings and bind during the arc of the suspension's travel. The SkyJacker JK longarm is the kit I saw. It used stiff urethane bushings, and had minimal flex for what a 4" longarm kit should have. Another I've seen is the Rubicon Express longarm. A buddies JK had the 4.5" RE longarm, and it would practically tear the bushings apart while flexing because it was binding so hard. These are all issues with bushings, not the length of the arms. With a quality flex joint, you likely won't have these issues. Eliminating binding is what allows you to take full use of the length of your shocks, to get maximum flex.
Someone please correct me if I am wrong.