I think both you and nurettin raise a very good point, in that FIX had some great aspirations, but the wiggle-room in the standards was too wide, and has resulted in, well, not a very a meaningful standard. The existence of certain companies that do nothing but aggregate and normalise broker and LP APIs is further proof of this.
Nonetheless, it'd be great to see brokers in the retail space normalise toward a uniform API. Even if that is a standardised HTTP interface with good WS support for streaming data, or a standardised binary interface. My feel is that FIX is the closest thing we have now that would get us most of the way there, even for all its warts. But of course, I'd be equally keen on anything standardised.
Edit: typo
