As per previous post, there is a lot of debate about what causes cramp, but little hard science to confirm that it is caused by salt deficiency. In fact, there seems to be some debate about whether salt deficiency is actually a problem at all.
e.g. have a read of this study abstract:
http://bjsm.bmj.com/content/40/3/255.abstract
Basically, athletes studied in an Ironman competition maintained their sodium levels OK with no supplementation.
Other suggested causes of cramp include fatigue and dehydration. So getting fitter and making sure you are properly fuelled up during exercise might reduce the problem. Substituting a carbo drink instead of plain water might help here.
Anectodally, cramps seems a very indvidual problem. I've never experienced cramp in a race (and have never used mineral replacment drinks) but know people who cramp regularly. On a few occasions I have noticed the odd spasm in leg muscles towards the end of long sustained race efforts, which might fit with the fatigue theory.