Basically, you get the reward for doing your job properly. Off-pitch performance is also important as players can be real club men. Some players see training as a chance to improve both themselves and the team as a whole. Others see it as unimportant and don't put the effort in. There would have to be compensation for when players are injured - as they obviously won't be able to perform on the pitch if they are unfit to play.
Unions are always strongly against what is, essentially, performance related pay.
But however they may hate it, in the end it rewards those who want to do well. Players may not be playing 100 per cent, but if they are putting the effort in then they will be rewarded. One of the main gripes the PFA would have with the system would be that performance is really opinion based. One person may have a different view on a players performance than another. This could be overcome through the setting up of an independent review panel, roughly along the lines of a industrial tribunal, which could hear grievances where players feel they have not got what they deserve.