To be sure sliders won't always save the bike, but that's no reason to not have them. Most of the time they will at least minimize the damage. It's just like having gear when you ride; the lid and leathers won't always keep you from dying, but they sure improve your odds, so just wear them.
My bike hasn't been down on the road yet, but has managed to be knocked over on both sides while sitting still (note I blame the bike, not the stupid owner

) and the only damage is a little (1/2 the size of a dime) scab on one side of the plastic by the passenger seat; woulda been way worse without them.
Check out the active topics and you'll see pics of a bike that went down from being side-swiped. The pics are great for showing how much sliders can help.
