>>>When the weather's bad you can still go to a lot of the indoor attractions such as the aquarium and various museums!<<<
And that's the crunch right there.
The visitors who purchase this card, are spending time - bad weather or not - visiting attractions that locals wouldn't really recommend that highly. Madam Tussaud's, JetBoating, Maritime Museum, even the Sydney Tower wouldn't make the Top 10 attractions of most locals I'd imagine.
And if you happen to skip them, then the value of the discount decreases significantly. You also have the inflexibility, in that you cannot take day trips once you've activated your card, as it has to be used in consecutive days.
Not convinced at all.