A collective pitch machine maintains a constant rotor rpm regardless (unless you are landing and spooling down). The advantange is if you are descending and need to add power, on a FP machine you have to wait till the rotor spools back up before it starts providing more lift, often the helicopter and the ground become at one with each other before that happens.

The only two disadvantages to CP are the rotor head is more complicated and the throttle/collective stick is more sensitive. Both of which are far outweighed by the performance advantages. Also a FP machine is not capable of anything but the most basic aerobatics.