Match fitness is quite important as that percentage rates how long the player will last in a game. For example a match fitness of 100 means the player should last the whole game and be fit enough to play the whole game.
A match fitness of 40 means the players isn't anywhere near ready to play a whole game and is probably best off used as a substitute in the later stages of the game.
As a personal rule I never have anyone in the starting line-up with a condition or a match fitness of less than 90.