Well, I've worked remotely for 16 years and in most meetings I don't get to see the slides until the meeting starts. Usually I can only see them via some conferencing tool. Sometimes I get a copy in mail the week after. So I think the IETF is already doing pretty well at making materials available, and insisting on getting slides far in advance of the meeting is beyond what most people get in reality outside of the IETF. I would call this a SHOULD not a MUST.