[mpweiher]

There is a huge difference between a fixed, constant set of keys vs. the keys being an open-ended set that depends on user data.

[duped]

Not really, this is a very minor difference that people exploit all the time to minimize the size of serialized data or make it more readable. This is a great example of bikeshedding.

[mpweiher]

Yes there is if you optimize for the former case.

If that optimization isn't for you, choose a different library.

If that optimization works for your use-case, it can make a huge difference.