Natural rebels? The sterility of absolute goodness - I am thinking a little of Blake and his resentment of the stifling effect of tradition and the establishment (of course for Blake
Orc is a positive figure.
Villains can be more attractive than heroes, especially perhaps to adolescents who are going through a rebellious phase anyway.
While I think rooting for Sauron would be a bit sinister, there are lots of Sarumans in the modern world - he would have fitted in quite nicely with the "Greed is Good" culture of the 80's. In fact he may well have been reincarnated as Gordon Gekko.
Also, Middle Earth isn't so black and white (and is a lot more interesting when it is not so). The LOTR is the closest to being black and white and even there those who are good often have to break the rules to "do the right thing" (Faramir, Eomer, Eowyn, Beregond...), Boromir and Denethor are not faultless but they are not evil.
The Silmarillion is packed with characters who fought Morgoth but were not "pure white". Feanor for a start is hardly a gentle and respectful elf.... yet noone could have hated Morgoth more. He like Thingol and Turgon could be accused of selfishness. Personally I find Luthien incredibly selfish since she is only does anything when motivated by her own interest....
Basically a character who is all white is as interesting as a blank piece of paper...