Even a setup reliever, if dominant, can be extremely valuable.
Les Lancaster was a 4 bWAR guy in 1989, as an example.
The next year, 1990, Rob Dibble was a 4 bWAR reliever setting up for Randy Myers.
Yup, fun examples, great memories! I think the "if dominant" is kind of a limiter, though? Being very good is one thing; being Hader, Dibble or 1989-Lancaster is kind of remote.
The other limiter with all three of Hader, Dibble, and 89-Lancaster is the usage. All three of those are multi-inning guys.
Dibble had 98 innings (98IP/68G): that's 30 "extra" innings.
Lancaster: 73IP/42G, 31 "extra" innings.
Hader: ~80 innings, ~15-20 "extra" innings per year.
The vast majority of relievers in today's usage are single-inning guys. Relievers who are routinely pitching "extra" innings and accruing 80 or more innings are very unusual.
*If* Burl ends up being so special that he's both dominant in the innings he gets; AND is so rubber-armed that his manager is going to throw him for ≥80 innings and ≥15 "extra" inning outings per year, that will be a VERY unusual and VERY special weapon.
I'm an optimistic Cubs fan, for sure, and I hope Burl ends up being phenomenal. But I think it's pretty unlikely that he'll be THAT special, or that the Cubs will have a manager who is unusual enough to be using him for lots of "extra" inning outings. It sure wouldn't be Maddon doing that, that's for sure.
Heh heh, I think Blue's original point was that unless he's so great that he becomes a closer, that being a good setup reliever isn't going to be great value. Lancaster's year was obviously a fluke. And setups phenomenal enough to be Hader and Dibble, they pretty much satisfied Blue's greatness critereon, and quickly enough become closers.