The way I look at this now, (since this thread is about 2 years old) is that 5-day bodypart splits put way too much emphasis on the upperbody and not even nearly enough on the lower body.
The majority of the average gym population has the mentalitly "I don't care how my legs look. I only care how my upper body looks."
So that being said, you then have 2 types of people:
1.) Those who neglect legs altogether, and just do upperbody. So they have a "chest" day, an "arm" day, a "shoulder" day, a "back" day..... and probably another "arm" day.....
2.) Those who heard that "doing legs helps your upper body get bigger" (which is a really stupid statement, and I'll explain why in a minute). So they do their 5-day splits, and one of those days is a "Leg" day. They most likely hate leg day, and they don't put nearly as much effort into thier leg day as they do for their chest or their arm day. If a day is to be skipped, leg day would be the first to go......Because after all, "legs" are just another muscle group.....right?
I see it all the time with the people at my gym and in my MMA class. They all have the "one muscle group per day" mentality, and the one out of the 5 days that they do legs, they always bi**h about it. I guess they fall into group #2 as listed above. For some reason they all hate leg day.
Now, I think that the whole mantra about "Legs helping you build you upper body" is a s***ty way of thinking. Yes, the statement is true to an extent....but not in the way that a lot of people think. The reason that "doing legs" is so effective for total body growth is because not only do the exercises work your legs, which are the biggest muscles in your body, but they also work all the biggest muscles in your upper body just as much. They are "core" exercises, and "core" in this case does not mean abs, it means the center of your body.....The place where all the power comes from.....The place that has hundreds and hundreds of muscles in it.
Also, by "doing legs" I'm not talking about leg curls, leg extensions, and calf raises.....I'm talking heavy squats, heavy deads, and variations of the two. Squats and deads are "full body" exercises. This is actually why I kinda get p***ed off when guys at my gym always ask me the question "You really like working your legs, don't you?" Or jokingly, "Let me guess. Today is leg day for you, isn't it?"
Now since WE all know that squats and deads are the #1 mass builders, because we are more educated then the average gym rat who know nothing more then simply what they have heard from other uneducated gym rats, it then doesn't make sense to me why anyone would think that doing a 5-day bodypart split would be more beneficial then doing a full-body routine or a squat/dead/bench split or even an upper/lower split....
It makes sense that you train movements, and don't think "muscle groups". Because when you start thinking muscle groups you get:
- chest
- arms
- shoulders
- back
- legs
If you're only spending one day a week on legs, and 4 days on upper body stuff......well you see what I'm saying???? 4 days on the upper part of your body, but only 1 day on the lower part of your body???? Makes no sense.
Why not do a 5 day split like this:
- quads
- hamstrings
- abductors and adductors
- calves
- Upper body
It looks silly doesn't it????
That's why I don't understand why people think that doing a typical 5-day bodypart split is a good thing. It is essentially considering the lower body to be not that important, and just another (pain in the a**) muscle group to work.
IMO routines should be built around squats and deadlifts. When designing a routine one should ask themselves "Am I squatting and deadlifting enough?" Everything else should be secondary to that. Of course the other lifts are just as important - OH presses, pull-ups, bench press, rows, etc.... But they still are not as good as the full body, ultimate strength and mass building squats and deadlifts.
MVP was on the right track in saying that by putting deadlifts on "back day" then you're at least getting 2-days a week of "Legs". Now, if you put power cleans on "shoulder day" too then you would then have a decent 5-day split, because then 3/5 of the days would be not totally neglecting the lower body.
Unfortunately to the average gym goer that see squats as strictly a "leg exercise" and deadlifts as "an exercise that looks dangerous for your back", and power cleans as "ummm...no I don't think so", they would have a split that looks like this:
5-day bodypart split #1 chest day:
flat bench
incline bench
decline bench
flies
cable crossovers
arm day:
barbell curls
preacher curls
hammer curls
dips
tricep extensions
skull crushers
Leg day:
squats
leg presses
hamstring curls
leg extensions
calve raises
back day:
pullups
seated cable rows
lat pull downs
rows
some other back machine
shoulders:
presses
side raises
front raises
bent over raises
First off, no deadlifts at all, and look how little emphasis a split like that gives to squats

. And that is certainly not a rare thing to see at all. In fact, I see people training like that all the time in my gym (oh, and most of them don't even do squats either).
Now if you make a 5-day split that looks something like:
5-day bodypart split #2 Legs:
Squats
front squats
SLDL's
calve raises
Chest:
flat bench
incline bench
decline bench
dips
Back:
Deadlift
Rows
Pull-ups
Cable rows
Arms:
whatever arm exercises you feel are necessary
Shoulders:
Power Cleans
Overhead presses
Side raises
front raises
This split would be a lot better the the previous split I mentioned. In fact, since it has 3-days that have a great "full body" exercise in it, It's not even that bad at all (however it's not my thing). But most uneducated people in gyms won't do that, and they will have a split which looks much closer to what #1 looks like.
So to conclude my rant, "Legs" should not just be considered another muscle group that get worked once a week like "chest" or "arms". "Legs" should not even be placed in the same category as "arms" or "shoulders". If you're gonna do a 5-day bodypart split, that's fine. I can't argue with results, and I know people get results with it. But please don't give 1/5 of your effort to the most important strength and mass building exercises, and 4/5 effort to all the "beach muscles".