Its not a matter of switching or using only one. They compliment each other. A PC can be used on newer finishes with not a lot of paint defects. For more serious defects I use the rotary. The PC can be used to finesse the job (like a fine polish or glaze.)
This is kind of a bad analogy, but a rotary is a rifle tackling down the big jobs, a PC is like a pistol to do the small jobs, and if both jams or are not needed, there is always the hand.
I have used a PC for 2 years, after a while I just needed a rotary. A PC can do what a rotary does with the right product, technique, and LOTS of time.
Most quality, pro polishes are actually meant to be applied by rotary (Meguiar's, 3M, Malco, Pro, Ardex, etc. etc.) Then lighter polishes can be used with PC (or a strong polish/compound may also be used.)
I use my rotary now on most cars to take out most/all defects, then use the PC to finesse the finish to a perfect shine. Then a hand rub down if I have time.