Well, are you talking from a player or designer perspective?
From a player perspective, you have no choice but to branch out oftentimes. The norm needs to be supported by game design, so the norm can only be followed if it is supported!
From a design perspective,
making everyone a little more equal in terms of power is the way to go. When one person expends the same resources pursuing one path of optimization, they should be as successful in that regard as another person putting the same resources into another path.
Example: the sneak should be able to steal, hide, and move about unnoticed as well as a tank who has put the same permanent resources into tanking.
The problem comes in when someone sinks less resources than a "specialist" into a style of play and is still as or more successful. You see an example of this with a tank fighter being surpassed by a tank druid, even when all of the fighter's class features are geared towards tanking. He can still never match a well-made druid.
To put it another way, in a point-buy system, people putting the same number of points into any one character goal should be as successful
and no more successful in achieving that goal than someone else with another goal. If the sneak can sneak fairly well, but the wizard can blow up worlds, and these two have invested the same number of points in their character designs, the system has failed.