The state of iOS photo management is a mess. In typical Apple fashion, the built-in tools work fine, but if you try to add anything else to the mix things get messy, fast. And in “anything else,” I even include iPhoto on the Mac. If you want to have be able to see all your photos on your iPad, regardless of what gear was used to take them, you’re out of luck.
If you shoot with both an iPhone and a regular camera, things get even worse. Sure, you can suck it up and use Aperture or iPhoto, but Lightroom is (for me anyway) way better.