When has Prospect suggested 'industrial action'?, in fact a common criticism each pay round or WPP renegotiation is the lack of action, ie do we keep our powder dry or not. To answer the question posed, by being in the union you have access to professional legal advice, I personally know of one person being defended from the bullet by a QC brought into the case by Prospect.
No organisation is perfect, and given that the bulk of Prospect negotiations etc are done by our colleagues, in their own time, I don't agree at all that the union has underperformed. It is defending our corner from a very determined employer, who if it had it's way would have us all working significantly poorer working conditions than we currently have regardless of what unit/grade you are at. There are parts of current deals I don't like, in fact they disgust me eg AAVA's when on A/L, unit banding needs looking at, how will they be solved outside the 'union' arena?