Yeah, you're still assuming the WACA to be a somewhat seaming wicket. We saw 'spin twins' at the MCG, Adelaide and Bellerive last year. Not traditionally regarded as spinners' havens and if the WACA is any easier to bat on this year (making it more like Adelaide), it might prove hubris to omit MacGill.
Last year we saw Symonds and Watson play with consideration given almost entirely to batting ability; perhaps rightly so, with 4 specialist bowlers, their role need not be major. However, batting form wasn't exactly prosperous. Watson need only be fit to play.
Clark was behind the 8 ball after the triseries. He'll be a lot further off with six weeks out, especially if his rival bowlers fire.