"Can we make him say 'flowering plant' instead of 'angiosperm'?" Yes. Open transcript in Word. Apple F. Find flowering. Find plant. Go to FCP. Load clip that matches transcript. Go to extremely approximate timecode that transcriber could last be bothered to note. Watch in flicky sound until you find flowering. Realise that inflexion is wrong. Repeat.
If this tool can take me immediately to every single instance of a word instantly, that's got to be a tool worth having.
I see it mainly as a way of tying the media to the transcripts.
In the stuff I do we rarely replace just one word, an that's mostly for ethical reasons; we'll usualy weave the commentary, if there is any, around what people said, the way they said it. Visuals will of course help in refining the point being made.
I could see a use for stuff like corporate stuff where many times the message has to be delivered the way it was scripted.