I've not yet seen a convincing argument against the following system:
LFR being available only to the previous tier, with bi-weekly lockouts to make it a more forgiving catch-up mechanic.
The most casual of casuals gets to see all of the content by the end of each expansion, and raiders don't have their sense of progression dragged through the mud. This should keep everyone happy.
So please, if you have a convincing counter-argument, do tell.