In Magic there are some games where you blow your opponent out and there are others where you get blown out. The trick is to win the ones in the middle where it could go either way. Cards that primarily effect that middle are most important. Other cards are "win more" or "lose less." Typically if you have Stinkweed Imp or Grave-Troll pre-board you are doing very well and very likely to win. If you have no dredger at all you are in a lot of trouble. Wasteland will just wreck you. One could argue that 0 dredgers is in the middle, but it is close to you getting blown out.
The important middle section is when you get that Thug or Shambling Shell. Basically if Shell/Thug is your only dredger, then getting 4 chances to dredge into something rather than 3 is a pretty big different and impacts the important games where things can go either way.
I don't think that's the right way to think about it, at least during deck construction. I think the right way to think about it is that you want cards that will turns losses into wins. So for example including more good dredgers is likely to turn losses (no dredgers) into wins (one Thug) without any sort of intermediate or medium state in between (because having a dredger is basically binary).