I'm assuming this is a proposal for a Tobin tax on each financial market transaction (not clear immediately from the website)?
If so, there are definitely strong arguments for such a tax - particularly if it can be agreed and brought in at an international level (although I believe the last govt tried unsuccessfully to negotiate this).
The strong advantage I think is not so much about revenue raising (although this would obv. be useful in current circs) but in the fact that it discourages purely speculative transactions.
So for example, if you need to buy currency to complete a real world business deal, or are buying futures to insure against the risk of a business input going up in price the tiny percentage should add a negligible cost (and other business taxes should/could be reduced to compensate).
But because speculative deals (ie buying currency with the aim of making money by selling it again when the price rises) generally work on such a tiny margin, the tax should make them much less profitable.
So essentially, if you believe that speculative trading in currency/futures/etc. is a bad idea, then a tobin/robin hood tax has a lot to recommend it.
BUT unless it was introduced alongside capital controls (of the sort that were very common until the 1980s, so we're not talking revolutionary socialism here) then it is hard to see how it would work on a one country only basis.