Hmmm, I'm not a fan, but I don't think it's possible to deny Mr. Bieber's talent. Teen/tween icon aside, one does not make it as far as he did without something there, there are plenty of pretty boys out there who would love to be where he is now but never make it. I defend a lot of mocked celebrities on the grounds of talent even if I don't care for them.
Otherwise, I do tend to agree that he is too "young" as a star where there should be a "vanity movie", especially seeing as backround wise he's nothing paticularly special. According to everything I've read he's family was never poor, he's not from a broken home, he has no criminal record, wasn't involved in any paticularly famous romances, and hasn't done anything that has been especially contreversial. At the moment he's not the right kind of material to try something like "Purple Rain", "Eight Mile", or other similar films.
At any rate as I get older I'm increasingly reluctant to diss celebrities I don't care for. Largely because I look at guys like Leonardo DiCaprio who were mocked just as heavily as a "Justin Timberlake" or other teen/tween boy singing sensation, albiet he was an actor. People called him a no talent hack who would go nowhere when he got older. Look at him now though, he stuck with it through the criticism, developed his natural talent, and now he's in stuff like "Shutter Island" and "Inception" and definatly a quality leading man.
Basically, while the movie isn't a good idea IMO, I don't think you can diss Mr. Bieber because of it, it's probably the people running him who came up with this idea.