If your code is compartimented properly, a lower layers (sub objects) doesn't have to have to do all kind of preparations just because a higher layer has "special needs".

For example, pseudo code in a sub-function:

if (that) write_field('that'); if (these) write_field('these');

With messagepack you have to go and apply the logic to count, then again to write. And keep a state for each levels etc.