Is there any practical reason to use quoted strings for JSON keys?

The real reason about why JSON keys should be in quotes, relies in the semantics of Identifiers of ECMAScript 3.

Reserved words cannot be used as property names in Object Literals without quotes, for example:

({function: 0}) // SyntaxError
({if: 0}) // SyntaxError
({true: 0}) // SyntaxError
// etc...

While if you use quotes the property names are valid:

({"function": 0}) // Ok
({"if": 0}) // Ok
({"true": 0}) // Ok

The own Crockford explains it in this talk, they wanted to keep the JSON standard simple, and they wouldn't like to have all those semantic restrictions on it:

....

That was when we discovered the unquoted name problem. It turns out ECMA Script 3 has a whack reserved word policy. Reserved words must be quoted in the key position, which is really a nuisance. When I got around to formulizing this into a standard, I didn't want to have to put all of the reserved words in the standard, because it would look really stupid.

At the time, I was trying to convince people: yeah, you can write applications in JavaScript, it's actually going to work and it's a good language. I didn't want to say, then, at the same time: and look at this really stupid thing they did! So I decided, instead, let's just quote the keys.
That way, we don't have to tell anybody about how whack it is.

That's why, to this day, keys are quoted in JSON.

...

The ECMAScript 5th Edition Standard fixes this, now in an ES5 implementation, even reserved words can be used without quotes, in both, Object literals and member access (obj.function Ok in ES5).

Just for the record, this standard is being implemented these days by software vendors, you can see what browsers include this feature on this compatibility table (see Reserved words as property names)

Yes, it's invalid JSON and will be rejected otherwise in many cases, for example jQuery 1.4+ has a check that makes unquoted JSON silently fail. Why not be compliant?

Let's take another example:

{ myKey: "value" }
{ my-Key: "value" }
{ my-Key[]: "value" }

...all of these would be valid with quotes, why not be consistent and use them in all cases, eliminating the possibility of a problem?

One more common example in the web developer world: There are thousands of examples of invalid HTML that renders in most browsers...does that make it any less painful to debug or maintain? Not at all, quite the opposite.

Also @Matthew makes the best point of all in comments below, this already fails, unquoted keys will throw a syntax error with JSON.parse() in all major browsers (and any others that implement it correctly), you can test it here.