Spurious reasoning mate. Different players have different weaknesses. Andrew McDonald did fine aginst SL when he played against them, as an example.
You pick your best bowlers. Siddle has had a lot of chances to be consistent, and he hasn't done so. It's not he can't bowl well, it's that he doesn't do it often enough (like Johnson).
Copeland played his opening series in fairly unresponsive conditions (first test aside) and not only bowled relatively well, he kept things tight, which is important too. He derserves to keep his place imho.
If Siddle, or even better Dougeh, was to come in for anyone, it ought to be Johnson, who has frankly bowled like a busted arse, Perth aside, for bloody ages. Again, it isn't that Johnson can't bowl - fmd, when he gets it right he's a force of nature.
It's just that he gets it right about once a year these days. Sad, but true. He's averaging mid-50s with the ball, I think, in his last half dozen tests or so.