When I started riding in the 80s, this was near gospel. And certainly the European pros were riding lots of LSD and you would hear about the importance of "base miles," there are certainly lots of people over the years who have achieved great success using this model. The other side to this were the traditional weekend clubbies who rode most of their miles over the years at L2 or LDS and just never got faster. Grab a group ride on any given weekend and you will find a large group that lives and rides this way. And the old greats trained that way.
I can't speak for them, but there are a large group of posters on this site, including Coggan, Stern, Alex, and Dave who contend that controlled research and the experience of more recent pros contradicts that time worn paradigm, and that LSD training is less efficient than L4 or SST interval work, even for riders involved in longer multi day events. I don't interpret them as saying L2 (LSD) work has no value, or that it doesn't have a place in a training plan. But to quote Dave: "But the ability to sustain power for hours on end is key and that's what L4,SST,Tempo work trains."
I don't think that there is a right or wrong answer here, only a difference in philosophies. It would be interesting to do a controlled study with a group of amateurs, one who rides mostly LSD for 12 weeks, the other doing L4, SST and tempo for the same period and seeing which group fares better over a 6 hour race. If you are coaching, I am sure that you have had success with your philosophy.
I do limited riding with my old group LSD junkies from my past life on weekends, but what I can say is that although I ride much less than they do-- I am faster even on the occasional century ride. I do practically no specific training for longer rides. The closest are 2-3 hour hard weekend rides that are closer to L3 than L2. Almost all the rest of the time is spent doing 2 x 20s and SST. Now one man's experience does not a controlled study make, and given my limited time resources (4 kids and a business), I really can't dedicate the same time to LSD riding that I could in my 20s so I can't say that I wouldn't improve from doing more.