I sideload 100% of my books with Calibre via USB cable. If I buy a book from Kobo (in rare cases), I still download it, strip it, and back it up. Whenever I use the native overdrive, those books are usually saved as well for sideloading.
However, MobileRead forum users are a small niche and are not indicative of most e-reader users and their practices. Most other readers likely are content to buy from whomever their e-reader manufacturer is (B&N, Amazon, etc.). The questions are poorly posed.
Also, your poll is missing. You may want to fix that.