My arguments against Watson being in the test team are as follows:
1, He's been played as a top order batsman over the last 3-4 years when he clearly has technique issues against the new ball, especially when it's swinging. If you're batting at 1, 2 or 3 you need the ability to leave balls that might go within a whisker of hitting your off stump. Watson doesn't do this. He sticks his front pad down the middle of the track and goes with hard hands at the ball. This is why he gets out caught at second or third slip a lot or LBW with ball ducking in (though in fairness, he's done a lot to mitigate that). He also has effectively no back-foot game. I'm not sure how you even get to test level in Australia without that.
2, He lacks the temperament to make a big score once set. How many times over the years have we seen Watson get off to a good start and then blow it by playing a poor shot and getting out. Four hundreds in 95 innings is a very poor return for mine and just as bad has been the ratio of 50's to hundreds (4 of 22).
3, He rarely makes runs when the team really needs them. The innings of 83* during the Ashes to guide the boys home has been a rarity in his career. So often he'll get a couple or a bit more of a start and get out, thus leaving the rest of the batting order to do the bulk of the work. He made a terrific century at The Oval in the last Ashes tour, but it was in a dead rubber when it didn't matter. Those runs might have been handy at Lords or Trent Bridge when the series was live. In essence, he's a bit of a flat track bully. He makes runs when conditions are in his favour, but rarely when things are tough (Warner is the same, BTW).
4, He gets injured a lot. It's less of a problem these days but it clearly still is an issue. It's hard to have a bloke in a pivotal position in the batting or bowling order who's not consistently available.
Now I understand and accept the argument that guys in first class cricket haven't put enough pressure on his place. That's very true. There comes a time though when you've got to try something different, especially in a team that up until about November last year wasn't going that well. OK we won 5-0 against the Poms, but Watson's contribution to those wins wasn't exactly massive. He averaged around 40, but look at the innings list. A litany of starts and middling scores, especially in the first half of the series when it was a bit hot in the kitchen. You expect more from a No. 3 batsman than that.
His game is excellent for the limited overs versions of the sport, just not for test cricket.