All-around or Specialist?

I have read or heard somewhere that people succeed in what they do because they either like what they do, or they are good at what they do. Usually, people like enough what they do so they become good at it.

But what about the concept of improvement, discovering where you are weak and working on it until it does not affect your total output or performance? They say a chain is only as strong as its weakest link.

Let us say you have 100% effort that you can put everyday on what you already have. How much will you put for your strengths, making what is strong even stronger? How much will you put for your weaknesses, improving what is weak so that you become solid overall and flexible enough to take on things outside your comfort zone? What will be a good strength/weakness ratio? 80-20? 60-40? 50-50? 20-80?