forbidden, forbidden

Another perhaps silly question:

Why are so many things explicitly forbidden in the table in section 4.4
of the XML spec (the table that summarizes where what sorts of entities
can occur, and what should happen to them)?

I'm just thinking about this as a programmer.

Why not just ignore some of these things, if they occur in a context
where they have no meaning or function.  Later, if you change your minds
about their meaning and function, you can change the spec.  And docs
that were once valid will remain valid.  Docs that were in error will
still be in error.

Richard