Julius,
It depends on the lifter as to whether you are prone to hurting your back doing them. I have pulled over 800 from well below the knees on a rack deadlift. I pull near 700 off the floor. I don't suspect I'll be throwing my back out anytime soon.
However, if you have a kid who can only deadlift 225 off the floor and he's pulling double that in the rack, then it's probably time to get a little stronger on the full range stuff first.
Silver is correct. It is a good exercise for lats, erectors, traps, etc., though honestly it is still inferior to full range pulls. But as long as you are doing full range pulls, then have at it with the rack pulls.
Matt