“Symetry” was mentionned, and I’d like to address that concept. Symetry is actually the wrong term (the correct one should be balance) since symetry basically means that the right and left sides of you should look the same. That having been said, we’ll take it for what you meant: having a balanced muscular development with every muscle being developped in proportion.
Well, good “symetry” differ vastly depending on who you talk to and what activity that person does. For example what is considered good “symetry” in bodybuilding is actually quite unbalanced compared to the ideal aestehtic or athletic body.
I’m not saying this to diss bodybuilders; I have trained many bodybuilders (including 4 IFBB pros and 3 who went to the olympia) and competed myself. But the reality is that to do good in a bodybuilding contest several muscles actually need to be out of proportion. For example the arms, especially biceps need to achieve a proportion relative to the rest of the body that is higher than what is traditionally seen as aesthetic, simply because big biceps create an huge visual impact in bodybuilding. Someone with overdevelopped hamstrings will also have an advantage because huge hamstrings are rare.
So if you go by “bodybuilding competition” standards, it is true that past a certain point it might be hard to stay “symetrical” doing only big basic lifts. Then again you do have several examples in the sporting world of athletes with big biceps who never do biceps work (gymnastcs, rowers).
If you use more traditional aesthetics and the athletic body as a measuring stick, isolation work is likely not really necessary unless you have a several imbalance and already a lot of training background.
I will also say one thing; I have trained athletes in 27 different sports as well as bodybuilders, powerlifters, strongman competitors, etc. Now I will go on record in saying that the best looking bodies that I’ve trained were a young strongman competitor, two Crossfit girl, a hockey player and a bobsleigh athlete. Sure they were not massive like the bodybuilders (even though the two crossfit girls would likely do well on a state stage without even dieting) but the overall look is much better. They are muscular, very lean and are very strong and powerful. No they do not look like comic book superheros, but they look better than the bodybuilders I train. Not only that, they maintain their look year round without severe dieting, anabolics use or the need to live like monks.
None of them do much is any isolation work.
Now people will point out that “yeah but they are puny, they don’t look as big as XYZ pro bodybuilder… I do not want to look like that”… the funny thing is that oftentime the people who say this are ligthyears away from the athletes physique they make fun of! It’s like someone who makes 20 000$/year making fun of a millionaire just because he isn’t a bilionaire!!!
One thing I noticed is that the AVERAGE athlete (average person training like an athlete) will have a better body than the AVERAGE bodybuilder. Sure you have some guys training for bodybuilding that have great bodies, but if you consider how many people train like bodybuilders, the success rate really isn’t high.
However if you go to a decent Crossfit gym, or where football players train, you will see that there is a much greater percentage of above average bodies than in the average gym. As I’m saying this I train people at 3 different Crossfit gyms and people from 2 others come to get trained by me. I also work with a football team (17-20 years of age) and hockey players… I also visit several commercial gyms every month. So I’m not saying this out of my AR*E 
Is that a preach toward Crossfit or athletic training? No… but it is to prove that you can build a VERY impressive physique doing only the big basics. Once you have build a super impressive overall physique and you find that some muscle groups are lagging, yeah, invest some energy in direct work. But the fact is that every single set you do in the gym costs ya… it costs recovery energy and you only have a limited amount to spent. My argument is that if you spend a lot of that “money” on isolation exercises you will not be able to spend as much on big basic movements and recover/grow optimally. It becomes a matter of selecting what is the investment that gives you the most bang for your buck when it comes to reaching your current goal.
If your current goal is just increasing overall muscle mass and strength, investing a lot of training money in minor exercises might not be the best solution. On the other hand, if you are already as big as you’d like, and find that your biceps are a tad small for your taste, deadlifting might not be the best investment either!
One last point: anabolics. I hate to bring this up because they are not just limited to bodybuilders. BUT in the context of this discussion understand that anabolics are basically a way to increase your “training money”… they allow you to recover and rebuild faster. So you can invest more money. If I make 250k a year I am likely to have the luxury of spending money on secondary things, whereas if I make 50k I will have to be more selective. Same thing with natural vs. enhanced training. If you increase your training money artificially you will not suffer from spending money on a lot of isolation work but if you have a limited amount of training money, if you spend too much on minor stuff you might very well shortchange your gains.
Bottom line is that isolation work is NOT necessary to build a VERY solid and impressive overall physique that would make 90% of the population super proud. But once you reach a certain level of development and need to improve specific parts, it might become useful.
