This isn't so much a rant as it is a ramble, but bear with me. I just want to share my thoughts and hope you let me know whether you agree or not.
When I started running my current MERP game, someone on another board asked how closely I'd stick to canon. I responded that the major events will stay in place, but the players would have a lot of leeway.
I know there are people who religiously stick to canon, Middle-earth fans included (But if you think Middle-earth fans are diehards about canon, you should meet the Forgotten Realms crowd!). I get it. People become unhappy when something they truly enjoy gets changed. But Middle-earth doesn't have one single canon. Some people use The Silmarillion as their canon; others prefer the History of Middle-earth series. Keep in mind Tolkien's Legendarium wasn't the only work that was constantly being revised. Look at all the times Star Trek, Star Wars, and even Doctor Who have been retconned. I can tell you a lot of fans were not happy about the changes made.
I've gone on record stating that canon is overrated. I don't think it's just a knee-jerk reaction to the aforementioned diehards or to all the changes made in my favorite settings. My solo MERP campaign had elements that deviated from canon significantly, but it didn't break the game. And since it was a solo game, it hardly mattered to anyone else anyway.
Back to the question I was asked. I said I'd give the players some leeway about certain things. I don't have a problem with someone playing a half-elf or a cleric in my Middle-earth game if that's what the player wants to play. The needs of a role player are different from the needs of a reader. A gamer needs to make a world his or her own, or the game may feel limiting to the player. Sometimes that means making changes (aka house rules) to the system. Other times it means changing a few details about the setting to allow things in the game system.
If you're a game master, you need to figure out what you want to keep and what to discard. Once you've done that, you should keep your decisions as consistent as possible. If you change your mind, talk with your group and see how they feel about any suggested changes. In the end, canon is what happens at your game table; everything else is negotiable.
Anyway, these are my thoughts on the matter. I hope you share yours as well.