You could use wire, indeed you could use a number of materials. However, rubber bands have a number of benefits - they are self-tensioning, they are discrete, so that if one fails, there are plenty of others to hold the wing on. Furthermore, if you are concerned about strength, just add more rubber bands, or use thicker ones (there are special ones for model aeroplanes).
Sufficient bands will hold up to anything you're going to try, and in the case of a poor landing (or "arrival" as it's often called) the rubber bands tend to give way allowing the wing to separate rather than causing serious damage to the airframe.