One can buy a good enough rear rack and pair of panniers for under $200. This puts the weight on the rear wheel instead of on your back. It also does not interfere with your clothing in terms of waterproofing or allowing perspiration to evaporate. Rucksacks are fine for kids going to school with a couple of textbooks and a sack lunch but hardly ideal for commuting.

I can put a brief case, street shoes, jacket, and other items in rear panniers without affecting my pedaling of the bike. At my destination I can unclip the panniers and carry them inside with their handles and I have the option of only using one of the panniers if that is all that is needed.