If Watson is fit to bowl then no they need the variation of Lyon. Might work in a match but not over a series. Only reason to pick 4 quicks + Watson is the fact they are really scared that Harris/Watson/Pattinson might break down in the match and if they think that then a spinner should always play as they can take up the slack anyway.
I think Australia will be mad to not play a spinner, look at the Headingley test v NZ. Swann got a huge haul because the pitch turned, the ashes pitches will almost certainly do the same.