You won't pay twice. You pay for access to the system. The applications just act as portals, as far as I can tell.
As for needing OS 4.5 for the voice commands to work: they work for me without OS 4.5. It records an .amr file and transmits it to their Nuance servers which translate it to text and then pass that along to the search. You can also send them voice feedback that way.
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