I don't understand how "download and keep originals" is different from what already happens without enabling iCloud Photo Library. Aren't my photos backed up to icloud now? It seems like photos and videos take up lots of my 5gb of iCloud storage. And if I take a photo on my iPhone it appears on my iPad, so isn't that via iCloud?
So someone needs to explain the difference between not enabling iCloud Photo Library (but with photo library set to back up under icloud in settings), and enabling it with the "download and keep originals" option.
The difference is with the iCloud Photo Backup, the only way to access your photos is by doing a restore on your phone. It's an all or nothing deal. With iCloud Photo Library, your photos at an individual level will be able to viewed and restored from a PC, Mac, device, etc. It will be a lot better!