In my day an MLB roster had a long reliever, a couple of middle relievers and a closer. Do I think 8 man bullpens and relievers that come in for one batter and leave are ridiculous? Yep.
So I think it is a weird rule, but anything to keep a manager from parading in RP after RP is ok in my book. If you aren't good enough to face more than one batter, you should not be a pro pitcher. Your arm won't fall off. Nowadays a SP gripes about pitching more than 5 innings. Suck it up, man.