Shares are cancelled when a bank is nationalised (which is different to the government taking a majority stake, like RBS in the UK, where new share capital is injected which dilutes the current shareholders). In a full nationalisation the government doesnt effectively buy the shares for zero - they are just cancelled and no longer exist, and one share is issued to the government.
In terms of former Anglo shareholders getting paid anything, it would just be a gesture of goodwill - there is no obligation to pay anything if the bank becomes profitable and has a positive net worth in the future. There is now only 1 share, owned by the goverment. To the extent that the government wants to dish out some value to former shareholders based pro-rata on their former shareholding, that is up to them. But in any case, as huskerdu says, the bad loans would have wiped out any positive valuation anyway.
Buying the shares is a punt on the banks being able to function once they are cleaned up. If you have cash you can afford to lose, the upside is pretty big.