Christie Malry asks:

1. Will the 50% rate actually raise revenue?
2. Is it moral for the government to take 50% of an individual's marginal effort?

My answer to (1) is: I don't know, and nor does anyone else. There's huge uncertainty on the behavioural response to higher taxes. All we can say, for now, is that data on house prices, FDI, emigration and the movements of Premiership footballers suggest that fears of a mass exodus of workers or investors seem - so far - to have been wrong, which corroborates the evidence (pdf) from a tax rise in New Jersey.
My answer to (2) is: yes, for three reasons*.
First, an individual is usually free to expend marginal effort - to work harder or to take a higher-paid job. This being so, he chooses to pay that 50%. In this sense, a high marginal tax is more moral than a high average tax; most of us are compelled to work, but we are not compelled to earn more than ''150,000pa.
Secondly, a high marginal tax on high incomes can be seen as a form of insurance payment.
If we were behind a veil of ignorance - not knowing our skills or demand for them - we might well agree some kind of insurance, in which the low-skilled get a payout whilst the high-skilled or lucky pay a premium. Most risk-averse people, I guess, would think it reasonable to pay extra if you're in the top 1% of earners in exchange for income support in the much higher probability event of being disabled or unskilled. (It's this sort of thinking that lies behind the slogan that the 50p rate is "fair").
From this perspective, the objection to a 50% tax rate is not so much that it's too high, but rather that it is insufficiently used to insure folk against bad outcomes.
Thirdly, there are circumstances in which we agree that it's right for the government to take 100% or more of an individual's marginal effort. If a man works an extra hour a day burgling houses or mugging people, we'd expect the state to take all the fruits of that labour.
And some high-paid work is similar to robbery, in the sense that it's zero- or negative-sum activity. The banker who takes high risks knowing that he'll get a bail-out if he loses, the middle manager who spends his time on office politics and sucking up to his boss, the manager who exploits his workers, patent trolls, (some) libel lawyers or lobbyists campaigning for special favours or regulations are all making money at others' expense. It is, surely, entirely reasonable for the state to seize the proceeds of such activity and to deter them.
Herein lies a difference between left and right. Whereas the left see the rich as mainly greedy exploiters, the right see them as public benefactors.
I don't know which side is more right.
Which brings me to a problem. Nor does the tax system. It treats the incomes of Fred Goodwin or Keith Schilling in the same way as those of James Dyson, Morrissey or Jack Wilshire. Money earned at others' expense is treated the same way as money earned whilst increasing social welfare. In this sense, there is an element of horizontal inequity about the 50p tax rate.
For me, this means that inequality cannot be tackled through the tax system alone. There must be other institutions in place which penalize zero- or negative-sum activity. (Clue - workers' ownership). One reason why I'm not a social democrat is that social democracy has, generally, failed to see this.
* Millions of low-paid people have for years faced withdrawal rates of far more than 50%, because they lose in-work benefits as their wages rise. I ignore this.