I can't even remember the last time I used my usb cord on my kindles. Now I only own kindles as e-readers and a Nexus tablet. I buy most of my books at Amazon. But I have purchased from other stores before like direct from Kensington (Discoveranewlove), AllRomance and got a few freebies from Smashwords. I sort it all in calibre, but don't use calibre to send anything to my kindles. I use the send to kindle plug-in and so those books are part of my Amazon Cloud.
No clue if that is considered side loading in the spirit of the discussion. Now I used to side load back in 2008 with usb on my very first kindle. I was still a bit confused at the time.
I don't really keep a lot of books on my devices, I pull them down when I need them and they reside in collections until then.