Re: enriched-mode and switching major modes.

[Richard Stallman]

    It might help if the copied text never crosses the boundaries of
    styles, i.e. if all of the copied text has the same style properties.
    Forcing rectangle copy could help us make sure such crossing never
    happens.

I don't think that limiting to rectangle copy would have that effect.
It is easy to select a rectangle that crosses paragraphs.