Should reindex serialize partial index and search requests

As part of https://github.com/elastic/elasticsearch/pull/22691#discussion_r98434085, we noticed some behavior that deviates from the normal assumptions of serializing requests when using reindex. @javanna said:

we always make the assumption that serialization relies on the request being valid according to validate I think.

Based on that statement (valid means validate passes), the way reindex serializes a search request and index request violates the assumption. Should we consider removing the use of these partial requests and just serialize the data we expect for reindex?