Batting averages show Parnell at 23 and McLaren at 19. Bowling averages show Parnell at 30 and McLaren at 27 (both average 1.4 wickets per match). There's therefore not much to choose between the two of them so hardly a case of a "bad career" versus "bad form".
Parnell does bring something different to the party by being a left armer with all our other seamers being right armers. I like McLaren but I don't think he would bring much that Parnell wouldn't.
Fact is though we are lacking a world class all rounder like we had in the past in the mould of Polly or Big Mac