Besides that, why exactly do you think assistance exercises are bad when accompanied by core lifts? Do you actually think the newbie is going to overtrain by adding a couple sets of lunges and RDLs after he’s done squatting? I doubt it. Especially if he’s doing something like 5x5 with only 135.
Here’s more of my reasoning. Lifting weights is just like many other sporting events. It requires a lot of coordination and skill to do well. If you are going to learn to play baseball, you start by playing catch, and hitting (usually off a tee) For 90% of people that ever play baseball, its going to come down to natural ability and just the basic playing catch, hitting the ball, etc. But when you get to a certain level, there are a ton of other drills and skills that will be necessary to learn to take you to the next level. You suddenly have to learn all the different strategies for different situations. You have to learn to hit different pitches, and field a ball more optimally. At this point very little of your training is going to be playing catch and hitting a ball (especially not off a tee).
The reason I use this analogy is because it is very similar to lifting. If you were to take someone that was just learning to play baseball, and have them work on learning to pitch, and hit curve balls, and sacrafice bunts, etc. before learning how to throw, catch, and hit off a tee; their progress would be exceptionally slow if at all.
Lifting is the same way. When you are first learning to lift, start with the most important exercises. Put all your focus into learning these exercises. Curls and leg extensions require little if any skill. You can pick them up and be good at them in a session or two.
Very few people put enough focus into learning the core lifts correctly, because they are more challengin and require more skill. And this is why nobody in the gym squats or deadlifts, and very few people can do pullups.
More is not better. A beginner, or someone who is weak in a lift isn’t going to get much benefit from doing isolation exercises with no weight.
Again, it comes down to what is FUNCTIONAL. Not in the balancing on BOSU balls sense, but what is functional for your goals.
If your goal is to put on size to your back, then your primary goal should be to get strong in pullups, rows, and deadlifts. If your goal is to get strong at these lifts, then you get strong at these lifts, or the lifts that most closly resemble them. If you cant do a pullup, then do a lat pulldown. Not a straight arm pulldown, or a curl.
And my reasoning for JUST the core movements doesn’t have to do with overtraining. Its about focus and efficiency. There probably isn’t any scientific principle backing this up, but in the real world this is how I believe it would go:
An individual only doing the core movements; more specifically something like 3x5 or 5x5 knows that they only have three sets to get stronger and target their muscles. They’ll put more focus and work harder, and consequently get stronger and better at the lift faster.
Another individual that has 5 sets of pulldowns, 5 sets of curls, 5 sets straight arm pulldowns, etc. They have a lot to do, and will likely put less emphasis on the pulldowns, which WILL be the most useful exercise for their goals.
Also, you could think of it this way. If you took the CORE MOVEMENT ONLY approach, you have a few better options. You could add an additional set of squats rather than doing leg curls and leg extensions. (so instead of 3x5 or 5x5, you could do 6x5 or even 8x5. More practice)
You could workout more frequently. Instead of hitting legs once a week, you could hit squats 2-3 times a week, which I would bet a lot of money would be much better for strength and size in a beginner than a bunch of exercises less often.
We could sit here and argue this all day, and it really is all going to come down to different peoples opinions. Ive offered my reasoning, so you can either agree or disagree. It doesn’t matter to me, this is how I feel.
***I’d like to add, that im finding something similar with myself. I consider myself an intermediate LIFTER, and im finding that the core lifts are where I get all my gains from. Right now, my assistance/isolation work is usually 1 set to failure for a PR, and this seems to be working pretty well. Granted I said I am a lifter, and not a bb’er, so at my current level if I were a bb’er I would need a bit more focus on assistance/isolation.