The main reason for alting is to test other classes or builds, In City Of Villains you would need 99 character slots just to create one of every powerset combination of the Brute archetype. So getting the right one for you is rather important as you are locked into that choice forever, this results in many low level characters being deleted and re-rolled time and time again.
I was very pleased to learn that in Rift you should only need to create one of each class, as all souls become available to you as you progress through the game. This reminded me of the Guild Wars approach, where other classes are unlocked as secondary allowing you to have a wide and varied pool of powers to choose from.
The other big reward with this system is that you don’t have to abandon your invested time in one character in order to modify your build slightly or to try something different. Plus being able to switch between 4 different combinations as easily as using a skill, makes jumping in and out of public groups more fluid and easier for those who play differently within a large group.