With the LIb Dems sinking in the polls, in the great majority of seats at the next general election the Lib Dem candidate will come third. All AV will do is to allow Lib Dem voters the choice of transferring their vote to the Tories or to Labour.
In fact, AV might actually contribute to a decline in the Lib Dem vote. Previously many Tories and Labour voters voted tactically for the LIb Dems. Under AV they will have less reason to do so.