I've had Yoshimura systems on all 3 of my race bikes and have never lost a spring ever, with my 1st bike having something like 6000 race miles on it. None of those systems had any type of sleeve around the springs, they used just the spring itself.
Are the springs installed in a way that they are only touching the bike at the end hooks, and is the spring straight between the 2 mounting points (not twisted or bound in anyway)? I would lean toward material used in the spring if everything is correct in the assembly of the system (especially if the springs are not touching the bike anywhere except for the end hooks). As for vibration causing the failures that would still be due to incorrect material in the spring or poor design of the spring itself. The silicon bead or tube around the spring would act as a vibration damper and help to reduce the resonating stresses to the springs - but this still is only a bandaid on the problem, the real problem would still be the material used in the spring most likely.