I am a big fan of Tolkien's work and the idea of myth in general. I don't think it is too fair to rag on Rowling for plegerism (however, I do not remember the specific moment you are refering to where she rips off Bilbo's exchange with Smaug). I looked it up on google and found a few sites that listed the places where she ripped off Tolkien, and I think it is a stretch most of the time. I think giant spiders are fair game and have occured in countless comics and b-movies. I don't even think it is fair to Tolkien to compare Rowling to him, as he is a far, far better writer. Rowling characters are pretty two dimensional (IMHO). I feel Tolkien's all have a good degree of depth and believability behind them. In one of the books Harry and his best friend stay angry at each other for hafe the book. This shows it was written by a woman. Only a woman would write to male characters not talking to each other for a month because they are mad at each other. In real life it would have come to fists and they would have been best friends again before the blood stopped gushing from their noses. Also all the parts of the book about Harry's attraction to the opposite sex are very akwardly written. No woman can really understand the feelings a young teen had towards the opposite sex, it is futile to attempt it.
I am not sure how well these books will hold up with time, probably they will be considered classics someday. I personally didn't feel they were that well written. I think their appeal is that they introduce imagination to a bunch of poor kids raised on the tube. When I was little I played Star Wars with my legos, I imagined this one was Luke, and I did my best to make this one seem Darth Vaderish, my imagination did the rest. Today they have Darth Vader and Luke lego figures. No imagination needed. Even the days of blocky Mario left a lot more room for imagination than the modern age of overly detailed video games. Any kid I have handed a copy of the Hobbit to, found the language far too difficult to consider reading it past the first page, yet they are no younger than I was when I first read it. HP is written in very simple language with very large print. The Time Machine is usually considered a kids book, yet we have to abridge it and simplify it to market it to our children.
I am going to attempt to introduce reading to my kids at a very young age, and I hope they have as much fun reading books like the Time Machine, War of the Worlds, Around the World in Eighty Days, and the Hobbit as I did when I was young.