Both George RR Martin and Terry Pratchett said they'd written their fantasy books, in part, to answer questions about Tolkien-type fantasy that Tolkien did not address: What does the day after the Dark Lord is defeated, and the party to celebrate is over? What would Aragorn's tax policy be? Would Gondor really exterminate all the surviving Orcs, even the baby ones in their cribs?
I think the 'heroic' style of fantasy, where the good guys never have any major vices or moral lapses, and the baddies never have any virtues or redeeming moments, gives you a cleaner, often more fun, story. But literature isn't just pure entertainment, it's got a role in the world of holding up a mirror to society and the world. History delivers a pretty clear message: Even where there were clearly more moral, or more morally acceptable, individuals and factions, it still wasn't nearly that cut-and dried. And if the 'heroic' style, especially where the audience is encouraged to see themselves in the hero / heroine, is a dangerous message if un-tempered by something more realistic: It's the kind of message dictatorships use as propaganda, to justify atrocities, and to get it's followers to go along with them.
It's also just plain unrealistic, hurts suspension of disbelief, and leads to weird notions. Look at all the uproar over the new Lord Of The Rings based TV series showing Orcs as having babies, and caring about them. Like... hang on lads, how exactly did you think you got full grown Orcs? OK, Saruman was breeding the Uruk-hai in pods in the Peter Jackson movies, but that's definitely not in the books, or how most Orcs arrive ' they multiply after the fashion of Elves and Men' is what the Silmarillion, essentially the lore-bible of Tolkien's world, says. That means male and female Orcs, sex, pregnancy and some sort of family of at least stable social units.
Or it's like wire-fu. I lived in a rough neighbourhood in a poor suburb for 10 years. That's not what a fight is like, even when the fighters are quite skilled. A fight is messy, ugly, nasty, cruel. Wire-fu makes it look elegant, cool, desirable. That's a really bad message.
All that said: There's definitely a place for the heroic fantasy, and you don't always want to come home and grapple with notions of 'who's the real villain' before bed. And GRR Martin does, I think, go a bit too far and ASOIAF does very into almost car-crash voyeurism territory. 'Everyone is a monster of some type, and monsters VS monsters is all there is in the world' is also a really, really bad message.
