If you don't use your back-ups occasionally, they can't be ready to contribute if needed due to injury, not to mention the regulars will be on fumes come August and September.
Don Zimmer was the poster child for not playing back-ups, which I still consider the major factor in the Sox collapse in 1978. Jim Rice started all 163 games that year. He was a big strong kid and could handle it, but many of the others couldn't. Carlton Fisk played in 157 games, but the insane part was he caught in 154 games and started 151 games behind the plate. Rick Burleson played in 145 games, missing 16 in July due to an injury. So he played in 145 (starting 144) of a possible 147 games he was available for. It was like that up and down the line-up and when the back-ups did play, they weren't ready.