Being in the JIB or SJIB has nothing to do with keeping your pay down and quite frankly that is an institutionally paranoid way of thinking. What these bodies do is set a "standard" for companies, so that employees know what is a fair conditions in accordance to they rest of the industry. When JIB/SJIB came about yes, their pay rates were less than some folks but this is because setting a standard which is at the upper level of the pay scale could force other companies out of business as they would not have time to adapt their business to the immediate hike in costs. Instead this gave employers time to catch up and raise their rates both charge and pay in order to come into line and hold onto the good guys that they have.
I've worked with people who will jump about employers because "that guy pays 10p an hour more" and to be honest, they are normally greedy, selfish folk who's attitude generally shows through in the rushed nature of their work.
Don't get me wrong Mr Enigma, Im NOT suggesting you are like this at all. It was just a wee sidenote rant