Is it bad to use this? I have some people who use too much CPU and I need something like this.. Could someone PLEASE give me more info on the subject about how its bad and good.
It is not bad or good. It is just a feature. With that feature you can restrict user precisely with memory. That feature doesn't help you with CPU usage but can help with memory problem (i.e. if some of your users eat too many server memory). Also, that feature restrict only php usage.