[amelius]

It starts to become a problem when the messages are very large. Consider the case where you need to send a big set to a thread, so that it can use it as a part of a computation. Conversion to JSON would be too slow, because every element of the set would have to be visited upon invocation.

You often want large computations to be performed in a side-thread (to avoid blocking the UI thread). It would be a pity if such computations couldn't take large data-structures as an argument, because large computations often take large amounts of data as input.