Actually one of my biggest concerns with practicing with old balls is the scuffs make them break like crazy in completely unpredictable ways. It's like trying to catch a 60 mph knuckle ball. I'm taking shots all the time.
My post was more in jest about some using only new balls or a group of balls dedicated only for pitching. Which in the end is fine. I was just making fun. To each their own.
To your point though, once the "skin" starts cracking or coming off then we don't use it to pitch. It does remain a hitting ball. The bulk of balls we practice with are pretty "broken in". New balls are rotated in as it's not uncommon to lose a couple every month.
One thing to keep in mind is that a pitcher is likely to throw two or three different brand/model of balls throughout a tournament (at least around here). My DD used to get picky because she preferred to use what "she liked" versus what was made available to her. Whether it be the travel team, school team, or whatever ball the opposing team threw in after the hitter fouled 2 or 3 out of the field.