It seems to me from looking at this chart from Cycling Peaks that the differences in training at L3 vs. training at L4 and L5 are not that great. If you count the number of check marks at the different levels there is little difference. What I am getting to is this, is the extra effort required to do L4 and L5 really worth it? Riding at L3 is fun. It is the intensity that I will usually settle into in all but the longest solo rides, unless there are hills or strong headwinds. Unless someone is very near their genetic ceiling is the extra effort really worth it?
http://www.cyclingpeakssoftware.com/power411/levels.asp
http://www.cyclingpeakssoftware.com/power411/levels.asp