Man works, and produces wealth from that work. A free man keeps all of his wealth. This does not mean he keeps all of the benefits from his efforts - if he chooses to freely trade, his trading partners also benefit.
The MTR is how much an external force (eg a government) takes away from him when he produced the last unit of work.
In Western social democracies, MTRs are typically greater than 50% for wealthy people. Socialists see this as a desirable thing - ostensibly because it distributes wealth, but actually because it punishes the rich. Regardless, only the very ignorant would deny that it takes away their incentive, and possibly reduces the total tax revenue due to Laffer curve effects.
More importantly perhaps, the way that distribution is done in social democracies means that poorer people have a much higher effective marginal tax rate (EMTR) than the wealthy. Subsidies such as child welfare, travel discounts, electricity usage discounts, government housing, car registration discounts, stamp-duty exemptions, medical subsidies etc are lost as people earn more income, producing an EMTR in themselves. Recent claims put the EMTR as over 75% for low income earners in Australia - a serious poverty trap. Some people have identified individual cases where the EMTR is over 100% - effectively relegating these people to a lifetime of relative poverty.
Taxing the rich at 50% has a serious effect on incentive. However giving the money to the poor in such a way that their EMTR is very high is far more serious. The government, in trying to prevent poverty, actually increases it.
The term 'fairness' is unlikely to have a definition which is universally accepted but, while many define fairness in terms of equality of income, others define it in terms of EMTR - the percentage of his rationality he is permitted to retain.
See