I'm not trying to be a smartass here but I worked in a bucketshop as a trainee a long time ago...and from that experience I know that you haven't made significant dough against a bucketshop...otherwise you wouldn't talk like that.
For everybody else just think about it this way. When there is no backwardation, no commissions and 24/7 trading, how is all this paid? How are you getting fleeced...because you are getting fleeced.
As far as the small account goes: The eMicro S&P has a Ticksize of 1.25$ outright and 0.25$ for spreads...how much smaller do you want to go? The entire index is just 3600$, so you would not even need margin...
As I said there are tax advantages for CFD's in some countries but besides that you can either chose trading against a single counterparty that can see all your orders and your positions or you can trade anonymously on a lit exchange.
And it's not that they are not "liquid enough for big institutions"...I'm not sure if you are even aware of the fact that IG markets is not an exchange...Nobody who has even the slightes bit of common sense would bet against a bucketshop