The accessibility issue is the one I'm most aware of; as in, they haven't figured out yet how to make things still comprehensible to screenreaders if they also offer the side panel on the left. I figure the technological aspects play directly into that.

It might just be for communities? I swear I came across something along those lines while I was setting up kittens_and_spitefic or summerofthe69. Still, since I can't verify it at the moment, I'll take that bit out for now.