frame sliders are NOT meant to protect the plastics... hence "frame". you have asked a good question. it is an added bonus if you find a set that can also save most if not all your plastics in a low-side (depending on speed and whatever exactly happens in the crash)... however... there is also the danger that if the slider pucks stick out too much... and you low-side... the bike may get caught on the slider and flip over (the bike will flip)... causing loads of damage. i have seen this... and it really SUCKS. you will notice that many racers on most levels either don't use sliders or use very short ones (take a look at colin edwards rc51 sp2 from wsbk 2002)... but obviously these guys don't have to worry about forking out $$$ for new body work, frames, etc. so... in conclusion... for the normal street and track day rider... it is 100% worth the investment.
so far folks have been very happy with the protection to both frame and plastics (depending on incident) from the following brands:
(forgive me if i skip a few brands)
later... nick - mri