1. It's no longer the standard due to better fonts etc now. Was once required due to type fonts being much the same for lower and upper case characters, and then just hung on for ages - social inertia.

2. Web based systems dump the extra space so you waste the time.

3. In a printed book that's properly justified it look extremely stupid and wasteful of space and paper as it often means extra lines and pages in the book.

I spend most of my day either writing stories or editing stories of other people, and I find it damned easy to read single space documents UNLESS the person has used some idiotic font that's NOT suited for easing reading. Sadly, most people use fonts not suited to easy reading unless pushed up into overlarge font sizes. When you get a chance, compare how ten point Times New Roman looks compared to ten point Palatino Linotype. The Palatino was designed for easy print reading while TNR isn't unless you bump up the size too much.