Pitchers need to stay in a rhythm and extended rest can be more problematic for pitchers. Consistent mechanic checks and throwing are what pitchers need. Extended innings for Sub Relievers are not that big of a deal if the pitcher is stretched out and has his arm in shape, which he should under the direction of the pitching coach, practice and innings logged.
Psychological component can become a issue as well if a pitchers rest to long and don't stay throwing. That being said if a pitcher doesn't look fresh or feel good throwing, then they need to be shut down immediately.
Also something I always teach my pitchers to help throw better is work fast. When the pitcher works fast and gets into a rhythm, it helps throw strikes. Gets the defense in a rhythm & keeps them on their toes, your catchers get in a rhythm with pitching sequence, the ump gets into a rhythm, and the opposing hitters have to work faster and this rushes hitters up trying react to the ball.