All of us are tempted to welcome converts and are delighted when people join the party that we happen, for the time being, to be a member of. We then dismiss as traitors, renegades or worse the people whose judgment goes in the other direction and leave the party we currently happen to be a member of and join another party. I think it is probably human nature to use different vocabulary to describe what is essentially an entirely analogous process.
I was quite shocked because I did not know that my noble friend Lord Foulkes was a secret believer in proportional representation.