Josh, and perhaps someone else, have asked me a couple of times as to what is the point to having power sources. What function to they perform? Whats the fucking point? Power sources seem to just mostly illustrate the starting point entirely from a "fluff" standpoint.
If a power is based on arcane, you know its magic-that-doesnt-come-from-the-gods. If its martial, you know that its just sheer physical awesome...or steroids. On the other hand if its ki then you know its dead (har).
I say mostly since it also technically achieves a mechanical purpose, as some items and artifacts specify a power if you use another power from a specific source (arcane, divine, just not
asian ki). The quickest example I could find is the orb of indefatigable concentration (AV, pg 94), which is an orb with a long fucking name that also has a property that triggers when you use an arcane power.
That being said, I have no issues with power sources not providing a larger mechanical disparity than that much. They did absolutely nothing in older editions, anyway. Sure, you knew that a wizard used arcane magic in 3rd Edition, but that just meant that his overall usage was more restricted than his weapon proficiencies and that he had to deal with shitty sub-systems and mechanics like spell resistance.
I think that power sources are useful for informing you at a glance where a class is coming from. Couple this with the roles and its a very informative pairing of words: you know that an arcane defender is going to use magic in some fashion to make you very difficult to hurt, and good at protecting your allies. As a player if you are attracted to specific character archetypes or concepts, this is great. Red Jason likes the divine classes, and it helps him easily identify which classes are going around supplicating themselves to flying spaghetti monsters.
Like roles, its not like they in any way restrict your character.