Questions on "WCAG 2.0 presentation" approach

hello Shawn and all,
At:
http://www.w3.org/WAI/EO/changelogs/cl-wcag20pres#approach
First bullet says :
"File Format: Provide in Open Office .odp version (since most people 
will use PowerPoint), and a simple HTML with embedded CSS version. Do 
not try to incorporate
it into Slidey or other HTML presentation format, because these may not 
be good tools for the average presenter or self-study user, and thus not 
worth
the effort."
I don't understand why providing open office format will benefit for 
most users who use Powerpoint.
I don't understand the second part of the paragraph that says that HTML 
presentations would not be a good tool for novice presenters. Or what do 
you mean with "slidey or other HTML presentation formats"?
I also don't understand why advanced users would all be able to use 
powerpoint or open office presentations?

Best
Sylvie