It may be wise to constrain a (group of) processes to e.g. 80% of RAM and 80% of CPU - but what if there are no other processes on the computer? 20% of the RAM and 20% of the CPU is not used. For example when I'm playing with a game, I'd like it to use every bit of resource to get that 90 frames per second rate. How does it differ from a fork bomb from the outside?