As far as I know, getting a kindle while owning an iPad (and the iPad version of kindle is good) is a good idea if
1) You find the iPad a bit too heavy for prolonged reading, and
2) The iPad does not work well in full sunlight - some kindle models are the same, but the monochrome ones easily can be read in sunlight.
That would be the main reason, if you find the iPad too hard to handle and read. I think you may also want to bear in mind, depending on how you use the kindle, carrying around the 500+ electronic device with its glass screen vs one that'a a lot less less expensive and lighter.
I'm not into ebooks so I've never used that aspect of it, but I do know others that have both an e-book reader and tablet, and the screen illumination is one big issue. For some it causes eye strain.