Had a 7 pin fixed for the last decade. This year, moved to a 3 pin slider (30, 40, 50). We will see how it goes, but as other have said, I think the multiple pin sliders are the best of both worlds. A single pin can definitely be good for deer. My dad had one that he shot many deer with starting in the 90s.
For elk, I think you have to have multiple pins. As others have said, longer then 50 yards, I should have some time and I shouldn't be shooting past that if I don't. The other thing for me, you "should" be more accurate when your pin is closer to the middle of your peep/sight window. It is important to be the most accurate on longer shots...so for me, it made sense. Also, with generally 500 to 550 grain arrows for elk hunting...a range finder and dialing in rather then gap shooting should be beneficial in the 50 - 70 range.
I could see having a fourth pin to get to 60, or if you want a 20 pin, etc.