A major disadvantage of the runoff vote is that it will change the way people vote a second time. If someone votes for Party A, believing that Party A can win the elections and form the next government and after the initial vote it is determined that Party A performed badly, then that person is unlikely to vote for Party A if Party A is in a runoff. People do not like to vote for a losing party.
It is very likely they would subsequently change their vote if Party A is in a runoff to Party B which is their second choice that they believe can win, or vote against one who they do not want to win, or not vote at all.This changing of vote in the runoffs, from their original choice of Party A even though Party A is in a runoff, would create a false distortion in the final voting outcome.
How can that be good for democracy?
Charles Thavenot,
Westmoorings