I think the best analogy is the colour wheel in Magic: the Gathering. Power Sources are cosmetic in the same way that colours in Magic are cosmetic.
Theoretically, there is no mechanical reason that stops Wizards from giving card-manipulation to white instead of blue, or direct damage to green instead of red. However, for design reasons, each colours has its own niche, the things it is good at and the things it is bad at.
Similarily with power sources, each source will probably have certain elements that it is strong in and certain elements it is weak in, but those restrictions are not really enforced at the mechanic level, but more at the design level.