If it wasnt necessary the never would have added to the game at all
But this is why they added it From Greg Street himself
From Wayback machine /20101107221842/http://www.worldofwarcraft.com/info/underdev/dualspec.html
I quote
"We really felt like this was a great way to increase the flexibility available to players and encourage them to take part in more aspects of the game. To use just one example, some players like to participate in both raids and Arenas, which is awesome behavior that we want to promote. But, there are some talents which are more useful in one part of the game than another. Currently, players have to pay respec costs and go through the process of setting up the desired talent spec and action bars whenever they switch. "
“The new system makes this a much more logical process, saving on time and cost and allowing players to easily move from one aspect of the game to another.”