I think it's because we're used to you guys going bananas over someone when they do well against weaker teams leading into an Ashes series (Bopara...Bell...Swann to an extent last time...there's quite a list), we feel someone has to restore some sanity to proceedings
There have been some very, very good spin bowlers to tour Australia recently and not go so well. If Swann does better than all of them then he will deserve to be heartily praised at the end of the series. He has played very well against SA, and did much better than our bowlers could manage against Pakistan, so he may well perform better than any other spinner recently has in Australia.
I find it understandable that people have their reservations though. I think this current Australian side will probably offer Swann and the other English bowlers some assistance, so he may well take a bag. There's a massive difference between saying, "I think he's good, but maybe not great" and writing him off too.