Personally, I love the lifts but if you are trying to get good at powerlifting, just do speed squats and pulls.
There may be a time early in a training cycle when you can throw them in, but if you are preparing for a meet drilling technique while pulling fast is important and stick to the spp lifts.
They are worth learning if you are a lifter. I suck at snatches, but I've power snatched over bodyweight and after that said forget this, but I still love power cleans. Nothing feels better to me than pulling a heavier weight off the floor, especially if you can get it to your shoulders then overhead. I need to do more of them.
They certainly won't make you weaker, but after a certain point, if you want to be a powerlifter you need to specialize in the squat, bench, deadlift.
I've noticed that power snatches are a nice back off to heavy pulling, but there is smaller carry over, mostly due to the snatch grip and greater technique required. Power cleans there is more carry over, but if you can power clean a speed deadlift weight (60-70%) you need to get your deadlift up or start competing in oly lifting.