I look at it this way.
The staff will pitch about 1450IP in a season.
The rotation will pitch about 800 of those. The bullpen is left with the other 650.
So they already are going to handle over 40% of the innings. Using 3 or 4 pitchers every now and then takes them out of the next game or two, which means the starters need to go more than their 5-6IP in those starts or use the recently overworked bullpen. Now with these early season off days, it worked out. But as the season goes on, it’s a bad idea to shift more IP from that 800IP bucket to the 650 IP bucket. Especially considering one is only expecting about 60-70IP apiece from each reliever, already leaving the non-rostered relievers to pick up nearly 150-200IP over the course of a season...