He absolutely did not beat Chael easily the first time. He even lost some of the striking exchanges, Chael even managed to knock him off balance a couple times. Weidman may not be a better wrestler but he is a better striker and a far better mixed martial artist who can chain his striking and wrestling way more effectively than Chael ever could. The results of the Anderson fights prove that
I did not say he beat Chael easily the first time.
The reason why the rematch is important is because I believe Silva said his knees were messed up when he fought Chael the first time (ditto with Travis Lutter). I recall hearing that right after the first fight, before anyone knew how the rematch would go. Considering how he beat Chael in the 2nd fight it gives more credit to the excuse (and he won the first one anyway, he didn't have much of a reason to lie).
Silva has fought guys with good takedowns before and has beat them. They usually can't take him down or can only do it for one round. Maia, Sonnen (2nd fight to be specific), Okami, Henderson are four guys with legit takedowns.
Silva vs Sonnen 1 is the only type he really struggled. But Sonnen is a specialist though, so it isn't really a sign of weakness if Chael Sonnen can ride you out for a decision.
Silva might not have stellar wrestling, but he is still incredibly fast, his reflexes are even better than his speed, and he has knockout power in "8 limbs". Not to mention even if you get him down you can't really submit him and his chin is granite, you have to opt for a decision win, and secure at least 5 takedowns (probably more).
It's not a promise at all that Weidman can take him down and ride him out.