1) It easily allows multiple "views" of a document (ie. you can have a
large text view and a small text view).
2) It will remove stylesheets from the HTML specification
entirely. They really are a separate issue.
3) It will ease development of both stylesheet languages and HTML,
because development can take place separately.
4) With no style information in the HTML, the data is more easily
reusable and far less application dependent (though I admit, CLASS
and <style> limit the overall effects a lot).
5) If the style information is separated out, there is a very large
chance that stylesheets will either be cached, or a local set will
be used, thereby reducing network resource consumtion.
There are probably other benefits I've missed.