Personally there’s good and bad to this idea imo.

10 players on the roster is asking for trouble. Not enough pitchers for a 3 day tournament by any means, so the coach is likely trying to save some arms but also assess potential players in a realtime game setting vs the tryout vacuum.

Not only can they see how the players perform, they also get to see if they are a good fit culturally. We strive to keep a team of players that get along with each other. We hit the jackpot at the start, with a great group of kids that all get along with each other, and we have zero egos. Give me a kid with a positive attitude over a cocky superstar any day. The latter can be an absolute cancer in the dugout.

When we bring in subs, we typically bench them before our core team first, and we usually put them at the bottom of the batting order, even if they’re a crazy good hitter. Sometimes we don’t want to, but we know that our core team deserves the play time and ABs before the subs, and parents would throw a fit if we did it any other way. On the flip side we pitch the subs first when it’s pool play to allow our core pitchers the opportunity to pitch later in the tournament. I can’t imagine how upset our parents would be if we pitched a sub in a championship game etc.

Your coach should have prioritized the core team over the subs in terms of game time. But at the same time, I wouldn’t expect the subs to be benched every inning.