I fish mostly rivers so I prefer a faster sinking line. If I had to wait a half minute to get to the bottom I would miss half the hole I am fishing. I use a Cortland 444 full sink type VI, works good for me.
I am however looking at buying a slower sink tip or full sink for fishing still waters. When I bass fish the smaller lakes I feel I am missing fish because my fly sinks past them to fast.
So there you have it, a reason to buy both.