Well, not really. For a start the Premiership has 2 more 'regular season' games than the Championship (38 to 36). So if the second Championship team was to win the play off against 3rd or 4th they'd go into the final having played as many games as 11th. If it was 3rd or 4th in the final, then they'd have played 2 extra games, hardly much suffering and I think it's right they should do the hard miles to get promoted.
Really, this is no different from a straight 11th v 2nd proposal that many vaunted. As I understand it, the SPL agreed to give a play off spot to the SFL as part of the merger. It was the SFL who wanted to include 3rd or 4th in the playoffs as opposed to a straight 11th v 2nd. Makes their league more interesting, yet doesn't really effect the top flight team. In essence the 3rd v 4th and winner v 2nd is part of the Championship season, just like the round robin post split games are part of ours.