We have to draw the line in the sand between AAS use and natural lifting. Because they are two different games physiologically, and require different approaches to maximize progress.
For more information on receptor repair and stimulus needed for AAS training, check out this thread:
Dharkam is one of the foremost authorities on this topic.
But we have many studies showing that training a muscle will renew those receptors. This is why why there is a synergy between androgen and training in bodybuilders while steroids only work to a limited extent in untrained persons.
Take home point...intense contractions during AAS use help to repair damaged/insensitive receptors, meaning that a battery of work is a more viable option...thus elaborate splits work much better for AAS users....and fill the pages of bodybuilding magazines.
While an elaborate split "may" work for most naturals, it's generally not needed. The beginning gains curve, as studied by Casey Butt, goes something like this:
Year 1 - 16 pounds
Year 2 - 8 pounds
Year 3 - 4 pounds
Year 4 - 2 pounds
Year 5 - 1 pounds
I think most of us will agree that beginners should not be placed on complicated programs. Yet they make the bulk of their gains during the first 18 months of training, if done correctly. In many cases, if starting at a normalized weight, and not beginning underweight, trainees will gain 70% of their natural potential lean body mass during the first 18 months of training.
Because training should be an evolutionary process rather than a random jumping from one program to the next, it makes sense for a trainee to abstain from excessive training splits during their first 18 months of training - for most. Not all. This is not a rule, but rather an observation and generalization.
After this point, gains are minimal. There is little return on investment for doing a complicated split. For a natural, the biggest factors in continued gains after this point are:
1) Continued persistence - getting their back ends to the gym and doing the work.
2) Continued resistance - adding progression in some form or fashion.
Adding volume for volume's sake is often seen as a solution during this time because bodybuilding magazines and broscience "common knowledge" push the belief that naturals can get as big as they'd like to get, that it only takes longer. This is tag-teamed with the foolish belief that a natural can "re-ignite" gains after they slow via the use of muscle confusion and swapping of programs.
Do splits work? Yes.
Are they used for the right purposes? Rarely.
Are they the most effective form of training? Generally, no. Not for the 80% of people who will never compete.
Most people overwork, lack a proper diet, add too much fluff into workouts, and don't focus insanely upon progression of weight. If they do things correctly, evolve their training over time instead of jumping into volume when gains slow, the average natural trainee will never need an elaborate split.
...hard work generally trumps all. So those that work hard and eat right will gain muscle pretty much no matter what they do. Also, part of training involves pleasure. Some like to train every day, and I would never try to minimize their enjoyment of the iron by telling them not to. On the other hand, beating your joints and connective tissue into oblivion with heavy weight once you have gained a good amount of muscle is not generally an effective approach with regards to training longevity. God knows I've tried...