It is neither one nor the other. This is not a question that can stand by its own but has to asked in the context of a goal to be accomplished.
For a body builder, it may be considered necessary to work on that one muscle to achieve overall symmetry.
When I find an imbalance, I may first try to strengthen through isolation before I try to integrate. Here it is a step towards improved function. This is how I personally view muscles isolation. It is not a goal but a means to an end. I believe that it will create dysfunction to only train through muscle isolation.