Since every moto accident is different, it's a big fat "it depends". They may do more harm than good, or they may save your moto from loads of damage. I've seen both instances, where the slider acts like a leveraging arm, and breaks the frame. I've also seen where the bike went down hard and fast, and slider got ground down, with the majority of the moto left unharmed.
I think a lot of it has to do with how it's mounted. It has to be strong enough to not snap off from a hit, but if it's too strong, it'll break the frame. The mounts I see for the Ninja, use a plate-type mount, and the slider attaches to the plate. So the slider isn't directly tied to the frame, and the mount is aluminum, so if it hits hard enough, that'll snap instead. If you must have sliders, make sure the slider itself is Delrin, or some other type of plastic. It'll slide easier, and will break off before the frame breaks(hopefully). Of course, that grade 8 or higher bolt is still there, in the frame, lol.
For me, it comes down to the simple fact that if you wreck while going fast enough, nothing will save the bike. Street bikes, even if no frame damage occurs, are rarely ride-able after hitting the ground at anything above 35mph. Even if the frame is saved, it can still be easily totaled too. I figure that if it hits the pavement, it's done, a write off. So, I'd rather run without having to look at the things. If protection is of the utmost priority, well, that's what stunt cages are for, lol.
