Also, new advances in each field mean that those who don't keep their knowledge fresh or their skills honed will slip back to the left (less depth of knowledge).
The balance of a team depends on the scope of activities required. Some activities require more versatility - it may be unknown what knowledge or skills are required, so versatility is key. Other situations call for the absolute best person in a particular field.
In a group which is under supplied in versatility, it may take multiple resources to fulfill a single task. This reminds me of many HP demos (apologies HP but you come readily to mind) where one person knows each product well, but not many people can string it all together.
A lack of specialist knowledge however can be just as bad. There might be no one with that can force things across the line and win a deal or close down an open question. Each team member may feel spread too thin or afraid to call for help from outside.
So obviously a balance between the two is required.
The superstar might seem like the answer. One person who both versatile and has in depth knowledge. They certainly would be in demand in many situations. With a large team, superstars might be a luxury, but with a small team they are necessary.
To make best use of the other players in the team, the superstar should be used to help boost the other members, turning the unknowledged into knowledgeable, and increase the versatility of other members. They should be used in the situations that require the flexibility first, rather than being just the first choice for each situation.
Holding them in reserve extends the versatility in the team. For those who play bridge, it's like holding back your high trumps until you need them. Using the person with the least versatility required means you can handle that next request regardless of the skills required.