When an athlete is doubtful to appear in a match (particularly in team sports), the question "should (s)he play or should (s)he take the day off" is often raised by commentators and fans alike.
Most sports fans, myself included, will probably respond by saying "what a stupid question." They may also add comments such as "I paid good money to watch this game and expected him/her to appear", "(S)he is getting a boatload of money and some of it comes out of my pocket. I deserve better", and "Shouldn't (s)he forfeit his/her pay for this match?" These comments may be a reflection of a consumer's desire to get good value for their money.
From most fans' point of view, it is understandable that they desire to see athletes appear in every (sporting) event they can possibly appear. I am not sure if the follow term is explicitly stated in an athlete's contract, but the athlete is expected to make him(her)self available to play in all games except under extraordinary circumstances or when the coaching/medical staff advices them not to. So, the fans argue, an athlete should appear in every match unless the athlete's physical condition is so bad that (s)he is unable to play or the athlete encounters an emergency situation. The fans can also claim that since they pay the same amount of money for a match regardless of opponent, they expect the team to treat them fairly and field the strongest team whenever possible. (In cases where a team charges more for admission to games against "top opponents", the fans may also expect the opposition to field the best team.)
For the teams' perspective, things can be quite different. While a point won against a powerhouse team is worth the same as a point won against a feeble opposition, most of the star players will go to great lengths to ensure that they are available to play when their team is up against a tough opposition; while most of them will opt to skip the match if the opposition is second-rate. The team may also want to protect its prized assets from getting injured or having a pre-existing injury worsen.
The fans' desire to see the best players appear in every game may also be related to competitive fairness. For example, suppose that the final two opponents of team A are teams B and C, where team A plays team B at home and team C away. Team A is stuck at its position on the table regardless of the results of the two games, while teams B and C are fighting neck-in-neck to reach a certain position in the table (to qualify for the playoffs or to avoid relegation). To show appreciation of the fans' support, team A fields its strongest squard in the game against team B and wins the game easily. In the game against team C, team A chooses to give some of those who had limited opportunity to play beforehand some playing time and loses the game. Team C ended up above team B in the final table.
Naturally, team B (and its fans) will cry foul over team A's decision to send the best players to play against the team while keeping them from playing against team C. While team A's decision is not fair to team B and its supporters, there is no rule that requires a team to play all its best players in a game.
This may be an argument in support of mandating each team to send its best available squad at the time to play in each game. However, this may not be a sound solution, as fans and teams may argue that the rule is too rigid and forces those unfit to play to appear in a game, a decision which may worsen their injuries. This rule may also prevent some hidden gems and lesser lights from making a breakthrough performance.
As long as sports is being played by humans, it is unrealistic to expect the players to play in every game. So perhaps we should just stop worrying about whether a player will play in a game and enjoy the match.