Maybe someone with more knowledge can help me out on this one: Why is it no problem at all for a starter to throw 100 pitches every 5th day, but a reliever cannot throw 20 pitches every day, adding up to 100 pitches every five days? I get that something will be lost to warming up 5 times instead of 1, but the starter throws warm-up pitches every inning but the reliever only once or twice per appearance, which should mitigate some of that. But these days it seems like it is viewed as abusive to have a pitcher throw even 3 20-pitch outings in 4-5 days.
Is it just empirical observation that RPs' effectiveness declines over the year with that level of usage? If so, would that be the case for all RPs or would some have more endurance than others? Is there some physiological basis for the difference in 5-day pitch counts? Help me out!
I don't have an answer, but I have some theories on where things are headed.
With a reliever, the concern isn't just the gross number of pitchers over a stretch so much as it is how often they're worked. If, say, Ryne Harper throws 10 pitches on three-consecutive days, he's probably going to get at least the next day off. I'm sure there are some fancy athletic performance studies that back that logic up. I'm not sure what the magic formula is in concern to pitches in an outing vs. simply having worked, but very few MLB managers deviate from similar line of thinking.
Texas was doing some really interesting things with Jesse Chavez in the bullpen during May and June last season that really caught my eye. There were a few periods he threw 80-100 pitches in a week stretched over 3-4 appearances. That experiment ended when he was traded to the Cubs, who used him more like a traditional reliever.
Down on the farm, there's all sorts of fun stuff going on. Most relievers pitch multiple innings and 30-40 pitch outings are not all that uncommon. For example, Cody Stashak has gone 2+ innings in 16 of his 31 appearances this year and 30 or more pitches in 14 of his 31 outings.
I think the big difference in ideology is that, while winning is desired in the minors, it's not really the only thing that matters. That shift in philosophy results in a little looser thought process with bullpen usage.
I think sometime very soon we're going to see teams blur the lines between roles. I could see teams building their staffs like this:
Five-man rotation: Everybody only goes 4-5 innings (70-80 pitches)
Four-man multi-inning crew: These guys always cover 2-3 innings per outing, get two days rest.
Four-man specialist crew: Single-inning, high-leverage guys and LOOGY types.
The expansion from 25 to 26-man rosters next year could be what sparks the start of this line of pitching staff building. This makes sense from a strategical standpoint and a fiscal one. So many starters fade late or struggle with that third time through the order, right? Makes sense to shorten starts then. It's gross to even go here, but owners will also love this line of thinking because it will devalue starters and I'd expect it wouldn't really boost the value of relievers all that much. Could it also help prevent injuries/Tommy John? Not sure, but I'd think so.
A team like the Marlins, hopeless and in a rebuilding phase, should be testing out something like this right now.