First impressions have quite a big impact on someone's perception of a player, and it takes quite a bit for someone's to change their opinions. Nani at first was incredibly inconsistent, often having one really good game then plenty of poor ones. People who don't watch him regularly still view him like that, and think that this is just good form or something. Same thing with how people judge how a player does over the course of the season, David silva was brilliant for the first half but non existent for the second, yet you still see plenty saying he had a great season.
Anyways, back on topic, it would be disastrous if he were to leave. IMO when fit and on form, he is the best winger in the world, along with ribery (don't count Ronaldo and messi as wingers obviously). He is better than robben for me, and even though Valencia was better this season, nani has shown that he has much more to his game than valencia (and he showed that in 2010/11)