"On the one hand why bother earning an extra £ if you are only going to make 50p"
See this is a massive over simplification. 50 p on earnings over £150,000 still means you get a large salary, and are able to afford far more than anyone else.
Also your "why bother" kind of goes back to the now discredited Laffer Curve. People earning high salaries aren't just motivated by money, there is prestige, job satisfaction and many other reasons for wanting to do a high level well paid job.
Ask yourself this, if the higher rate of tax is raised by 5p in the pound, how much do you have to be earning for that 5p to make a difference to your lifestyle or to what you now can't afford.
At the same time it should also be made illegal to bring forward or manipulate income to avoid tax. The way things are, the richest pay the highest level of income tax, 1% pay 11% , but they pay an over all lower proportion of their income in tax, when you include purchase taxes.
This is because the wealthiest have a lower propensity to consume, as pointed out, how much worse does your lifestyle get from 5p in the pound more from every £1 over £150,000?
How else could we spend the billions raised, assuming we cut the loopholes, it brings in more than the £350m a week so beloved of the right when they wanted to leave the EU.