I don't think anyone 'expects' you to predict anything. There was nothing wrong with suggesting that (for example) Martyn would have a good Ashes, because all the signs pointed to it. The point is that even though it's better to have an opinion than sit on the fence, it's a bit rich for anyone to respond to criticism by implying that they are always right, and you're not really doing yourself any favours there.
Personally, I've been a long-time backer of a few players that have been widely criticised on here, like Lee and Watson for instance, and enjoyed their recent success all the more because of it. I was certainly wrong about, say, Pietersen though, who I didn't expect to have quite the impact on test cricket that he has.