AV doesn't change things for the better over plurality systems except to provide evidence of people's preferences

The alternative vote system offers no advantage over first-past-the-post, when we assume that the problem we seek to remedy is that of tactical voting. We vote tactically when we know that voting for our preferred, smaller party will not yield results, so we choose the least disliked of the likely winners. A voting system which is better than FPtP would seek to remedy this problem but AV fails to do this.

With AV we have the ability to put in order, according to our preference, at least some of the parties (or candidates) that are standing. We are able then to reserve until a lower preference the party which we consider to be our least disliked or 'insurance' choice. This would be the argument (to be made) in favour of AV but if we are sure that only our insurance choice will count, there is little point in casting higher preferences for other parties. It is only for cosmetic or rhetorical reasons that we would bother to vote for other parties ahead of our insurance candidate if we know that they do not stand a chance; it might be useful to point to the lack of proportionality in the system (as reflected in the first preferences that do not result in representation) but we have no more ability to alter the outcome than under the existing (plurality) system.

AV is useful only in so far as our ability to register our disapproval of the main parties is enhanced, it doesn't provide any greater means to have a meaningful impact on the overall result.