The thing is, all energy in the real world IS convertible to each other, albeit with losses at the conversion. While it would be incorrect to say that "electricity" is the universal power source, it is fairly trivial to convert mechanical energy into electrical energy. That's how a those flashlights that you shake to recharge work. Or heat energy into mechanical energy. That's how a car's engine works. Or chemical energy into electrical signals. That's how that science trick where you power a lightbulb with a potato works. In the real world, it's all "energy" and you simply have to convert it from one form to another. Heck, as Einstein proved, even matter at a certain point is just energy. E=MC^2 after all.
In minecraft, however, being an artificial world, we don't necessarily have these conversions. That's what a universal energy API would do. It may not actually provide the blocks that do the explicit conversions, just like physics doesn't actually build transformers for you, but it would provide a stable and coherent framework to allow different mod-makers to build those conversion blocks without resorting to hacks or guesswork.