1. You have given up on any sort of consistency.
2. The database implementation will never be able to optimize a lookup workload which is pretty close to pessimal. Not caring about 99% is only viable if can actually avoid the other 1%.
what is a document? How is an ORANUM or a bignum not a document?
One motivation for creating documents is that modeling document contents as relations requires the creation of a bunch of primary keys which no natural definition. A simple document might be an ordered collection of paragraphs, [p23, p57, ...]
Modifying such things is difficult. In fact, the most effective way of structuring modification seems to be OTs based on document offsets. What Google docs does.