Thanks Muck. That is exactly what I'm driving at. I do still work my clients' arms from time to time. Usually after the major lifts are over. Not on everyone though. Not even myself.
If you want your arms to be out of proportionally large then you may do some of those isolated movements. It really depends - I guess - on what kind of body you want to develop. If you favor the ripped, balanced physiques of the typical Men's Health model, follow my recommendations. If you want to look like a Muscle and Fitness model, focus on your extremities and get on some gear, and hope that your genetics were blessed enough to not get serious injuries from the muscular imbalances you are creating in your body.
If you force your body to constantly work in isolated movements, it will possibly "forget" how to work in harmony with the rest of your body (As Lou Schuler says in his latest book). How often do you see a professional bodybuilder shifting over to a professional level sport like football, baseball, basketball? I don't think I ever have. That's because their body is all show and no go. Don't get me wrong, they are strong in specific lifts, but you won't see any of these guys entering the olympics as gymnasts, sprinters, even shot-puters. Train as an athlete and form will follow right on the heals of function. You will have an agile, flexible, injury-free body that looks like a greek god, not a charicature of something that vaguely resembles a human (thinking Coleman here).
In real (3 dimensional) life, our bodies are designed to do only 6 basic motions: Squat, dead lift, lunge, push, pull, and twist. If you train your body with these 6 basic components every part of your body will grow (including your arms) in PROPORTION to the rest of your body. If you arms seem lagging right now, look at your program and see where you are not following these basic guidelines. If they are the weak link in then chain of muscles needed to complete a complex compound motion, they will receive adequate stress to force them to adapt by growing.
Those pro football players who have massive arms are in great shape often in spite of themselves. In other words, genetics play a huge role. Tell me one single movement that they do that requires them to have big biceps? Just because some of them do nothing but arm isolation and are great athletes does not make them fitness experts. Not even close!
When it comes to the mistakes I have made in my early days of training (having come up through the old bodybuilding gyms in the 80's), I screwed my body up pretty bad from training like a bodybuilder. I wish I had even heard the advice I am giving you now so I might have possibly side-stepped all those shoulder surgeries and back problems I have dealt with over the years. It took me a few years to unlearn those old 80's paradigms, which is the mainstay even today of the "Weider Principle", and surprisingly is still practiced in gyms around the country. While you are still so young, try to be patient and pull back and take a look at your long term goals, then develop a program that will help you get there without hurting yourself. Don't just run out and start lifting heavy things all willy-nilly or you will wind up injured.