1. This law makes it harder for governments to run deficits when the economy is healthy. This is a sound approach to the public finances whatever you think the government should do during recessions. Both the Clinton and Blair governments ran surpluses for part of their time in power and they are usually praised for doing so. It's hard to think of a good argument against that. Keynes said in 1937 that, "The boom, not the slump, is the right time for austerity". 2. The law will not prevent deficit spending during recessions. The point is to make deficit spending the exception, not the rule. That also means that deficit spending is much easier if we think we need it – it's easier to go from 0% to -5% than from -5% to -10%. According to Keynesian theory it is the change in spending that matters, not the level. Advocates of fiscal stimulus should love this rule – it makes their policies much easier to implement in busts.
3. Yes, the law can be repealed by an Act of Parliament. So can any other law, that doesn't mean that they're irrelevant. There is inertia in politics and a government that is seen to repeal this kind of law will need a good reason for doing so. Making the public more aware of what's going on with government spending makes politicians more accountable.
4. Even if our models of economics told us that it was better for the government to have as much flexibility over spending as possible, our models of politics tell us that constitution-like rules are a good way of stopping abuses. This is about political economy as well as economics.
5. An honest Keynesian argument would be that the public is too ignorant and will oppose necessary deficit spending, so it's better to keep them in the dark. In this case the argument is simply that monetary policy is clearly and demonstrably just as or more effective than fiscal policy during recessions and depressions (indeed fiscal policy probably only 'works' through the monetary policy channel). The US cut fiscal spending by $85bn/year in 2013 (the "Sequester") which people like Paul Krugman warned would cost 700,000 jobs. Because monetary policy was accommodative under QE3, offsetting those cuts, this did not happen.